We drummed up interest in creating real-life players over the summer, filling in those players that debuted from 2008 onward with MLBPP 2008 representation. There are still quite a few left to do - at least 30-40 that had significant impact on baseball from 2010 onward. I thought I would jot down my process in quick form.
The SetupGetting the beginning pieces correct is fundamental. If you spend 5 hours creating a guy, then find out you misspelled his name, it's all a waste. You can use wikipedia, baseball-reference, or fangraphs to get a simple snapshot of the player - most importantly their full name and handedness for batting/throwing. Never assume this data - I was just working Doug Fister, who pitched in the AL his whole career, but bats Lefty. Important trait! Also, these sites have a picture of the character's face so you can get the right skin tone out of the 5 selections. They roughly correspond to ethnic heritage (european/caucasian, asian, latino, light african, dark african) but give it a good look and make your best judgement. Remember Carlos Pena and how 'off' his appearance is in the game? That can annoy people.
Picking a player's main position can be tough - sometimes guys play out of position, or have split positions - or got moved to a position they never played before, like Daniel Murphy and Neil Walker. There are a few ways to do it, but I typically put a player at the position they've spent the most time in while in MLB. This meets with people's expectations - they can import the player and put him at his main position. But try to spend a few gambles to get secondary positions when they matter.
For pitchers this is easier - if a guy has been a reliever, but has also started, like Alfredo Aceves, Brandon Workman, and so many others - go with the one they did more. If its a tie, then its up to you - maybe you want to represent them as they were in 2008, or maybe as they are today. Both are fine - I generally use 2008 as a point of reference, since the goal is to have people import the player and put him on his correct team as of 2008.
Next is selecting the batting stance or throwing motion. This is easy to figure out - video highlights are everywhere, so just find your player and get a good head-on shot of him batting/pitching to see how he does things. For pitchers, delivery is important - most will be three-quarters or overhand, and the two honestly look pretty similar. Try to take certain information points in - where is his glove when he gets the sign, does he wind his hands over his head or not, how far does he fall off the mound - and then find a match. Batting stance is the same - note where the player holds the bat, if he crouches, how far apart his legs are, and how he kicks before the swing - then find a match.
None of this has to be dead-on - finding a 100% match is tough, even when you have every form available unlocked - just have it be a close approximation. I usually spend 5-10 minutes on this at the beginning - it means I can't reload for a great start, but I know my foundation is solid.
During Success ModeI won't cover the middle part - there are innumerable guides on this. With realistic players, however, the goal is usually to AVOID an abundance of blue abilities and super-powers. It's nigh impossible to make it through success without a few, sure, but make sure they are meaningful and tied to this player - and then adjust elsewhere. I basically use the justification rule - when I got stuck with Power Hitter on Daniel Murphy, I hated it, but then he did lead the Mets in homers one year...so I accepted it and left his power a few points lower than I might have otherwise.
Because you may end up with blue abilities or hot zones that aren't perfect matches for the character, and because our Success mode players tend to dominate a bit anyway, AND because the game starts off in 2008 and some of these players didn't debut for years, you will see I am CONSERVATIVE when handing out abilities. I don't want everyone having 200 power or 15 speed or 15 contact; ideally, these guys would develop for a season or two in the minors, then get the full-time job. MLBPP doesn't have that sophisticated a growth pattern, however, so it is what it is. I try to get MPH and SPD spot on with reality, since those rarely increase during a career. Other abilities, like pitch breaks or contact, I might go a point or so under where they are today, to allow for growth, or the fact that these guys may not stay at an elite level for long.
Players can deflate and lose their everyday role pretty easily - when we created Colby Rasmus, he looked like an all-star, so we gave him D contact and B everything else. Nowadays he fights for playing time. So if you have a decision to make between a 9 or 10 rating, go for the 9. If someone plays out season mode with him on their team, they can practice his skill up to where they want it to be.
EndgameIt's happened - you've spent a few hours (or maybe days) on a guy, his abilities are done and you made it through the frustrating scout/coach/spy-friends/dead-parents-resurfacing-and-working-against-you storyline. Great. Now it is the MOST important time to make sure everything is right! If you did all that work, and now get the DOB off by a few years, well that isn't going to make you happy!
Jersey number is cosmetic, and can be changed or set later when a guy is imported to a team, so its not too important. Still, if you can, choose the jersey number that player has worn most often. Some guys will keep the same number from team to team, others will bounce around.
Date of birth, again, can be found on wiki, fangraphs, or B-R. The month/day is good, but the actual age is important. I set all my guys to the age they were as of 2008 - so if I import them into a beginning Season Mode, they will be the actual age they were in February/March of 2008. I got one wrong one time, so I know the pain of putting in all that effort just for a silly mistake.
Eyes - most important here is getting rid of the big anime eyes and using regular eyes. Look at the player's pic on their bio pages - some guys have squintier eyes, some guys big open eyes, others more laid back, half-closed eyes. Approximate! I stick to the first 10-15 choices since they get weird after that, replacing the whole head. There are only two eye color choices, so if they have light green/blue eyes, go blue; otherwise, go brown.
Glove and Bat - I google images of the player, and find what color glove and bat they are photographed with most often. Some guys always use the same exact shade, others mix it up, so just go with whatever you find most common in the pics. No one is grading here, but the realism helps!
Wristbands - looking at those same pictures, how often did the player have a band on his wrist or forearm? If he constantly wears one, same shade, same arm, then throw it on! Chris Johnson wore a black band on his left arm in over half the pics I saw, so I added it to him. Otherwise - don't!
Hair - guys change styles all the time, so again just go with what you see most often. Some guys have real weird facial hair, big beards and stuff - or mohawks under their ball caps. Most don't so stick with the normal hairstyles at the bottom, in the normal human shades of brown to dark brown. Redheads and blonds are rare, but if you're guy has it, do it up!
Other - don't go overboard, adding shades or eyeblack or earrings or anything...those things are rare. Only if your guy has them on all the time should you add them. Its too bad tattoo options aren't in the game, since so many guys are covered with them now, haha. Keep it real...the more your player blends in with the hundreds that are in the game from the start, the better.
When you're all done, take a picture or two of the guys' abilities, then the password, and upload them (in cropped, resized fashion) to the board. I'll add to the wiki, and guys that discover this game in 3 years will be able to find your post and import your player. Plus, I'll import him into my game, with my eternal thanks.

It's really a shame that height or fitness aren't things you can set, since that would differentiate these guys more. I'd like to see Bartolo Colon and Pablo Sandoval appear thicker than the average player, and guys like Doug Fister or Randy Johnson appear to tower over the average player. But until Konami issues a 2015 or 2016 version of the game, this is the closest we get!