How do you set your defenders to mark a certain opposition player?
Welcome to the site mate, I moved your topic to tactics discussion, please try to create new topics in the right area thanks.
To answer your question, in FM, on the match day screens go to your tactics > player instructions > show instructions > and select which players instructions you want to view. Now tick the box that says "marking" and open the drop down menu to the right of it and you can select a specific player.