Roster Editing

Roster editing is generally a straightforward thing. You can download a save editor to do the work or, if you're insane, you can do a lot of it through PES.

Editing with PES
If you have a lot of patience/are in no rush PES can technically be a roster editor. You can edit your players statistically, and even in terms of appearance, if your character is entirely human. You'd probably be better off using a save editor though. Seriously.

Editing with ejogc327's PES 2018 Editor
Open your save file with the editor. The program automatically puts you into the player editing section, so navigate to the Teams section for a brief moment. Find the base save team that has the ID_Team value that matches your team's ID. Navigate back to the Players section and select the "Clubs" tab on the left. Find the team that matched yours, and you will see their roster listed in the column on the left. Select the first player in the list, you will see their attributes listed in the large box taking up the majority of the program's UI. Now before we go over the quick ins-and-outs, remember the two most important rules: YOU MUST ALWAYS APPLY YOUR CHANGES BEFORE MOVING ON TO THE NEXT THING TO EDIT. APPLYING YOUR CHANGES IS NOT THE SAME AS SAVING THEM So remember to press F5 before moving on to a new player, and press CTRL+S before exiting the program. Note that pressing F5 to apply your changes will put the editor back to the player list. Simply click the Club tab again to return to the team you were editing.

General
Important bits that can be edited:  
 * Player Name - Enter the name
 * Shirt (Name on Back of Kit) - Will stay in all caps, don't make the name too long here
 * Country - Set to Other
 * Height - Check Rules and set the height
 * Weight - Always 80, according to current rules
 * Age - Always 24, according to current rules
 * Foot - Dominant foot, one the player prefers to use in-play.
 * Playing Style - Select from the list or leave on N/A. See Player Cards for info
 * Position - Select position
 * Weak Foot Usage: Check Rules and set
 * Weak Foot Accuracy: Check Rules and set
 * Form: Check Rules and set
 * Injury Resistance: Check Rules and set
 * Edit Player: MAKE SURE THIS BOX IS CHECKED

Abilities
Based upon their player designation, set the abilities according to the Rules. Rather than edit each one individually though, use the 'Abilities Adjust' box below and to the right of 'Abilities'. To use that box, enter the number you want each ability to be set to in the text box. Select GK to affect only the GK-specific stats, PLA to affect player-specific stats, or both to edit all. If you select the '=' button, the editor will set the player's abilities to the number you've entered.

Motions
Selects the animations that player will use. Includes goal celebrations, so view them using the in-game editor first if you actually want to be choosy with them.

COM Playing Styles/Player Skills
See Player Cards for info. Pick which ones you want.

Positions
Make sure the only selected option is the position that matches the one you set in 'General'. The buttons have three settings, Off, 1, and 2, with 2 having the brightest/strongest color, differentiating from setting 1.

Rating
Nothing to actually edit here, just make sure the position in the dropdown matches your player's given position. Don't touch the things underneath this box. Do this for each player on your roster. And remember the two rules: YOU MUST ALWAYS APPLY YOUR CHANGES BEFORE MOVING ON TO THE NEXT THING TO EDIT. APPLYING YOUR CHANGES IS NOT THE SAME AS SAVING THEM