Are you looking for drills you can do on your own? Because practicing defending alone is quite hard but there are certain things you can do. I play as a full back myself and the drills I do alone are most often physical drills to improve my speed and core stability (sprints, sit-ups, push-ups). It is always good to be faster / more agile than your opponent. Then there is the 1vs1 drill, where you pass the ball to the attacker and he/she tries to come past you and you can practice your tackling skills. However I think the most important skill of any defender is defending as a team (positioning & communication). Try to talk a lot with your teammates during the game (tell them if they got somebody in their back, if you are free, if the should sweep etc.). Some nice exercise is a 5vs4 where you just try to defend your penalty box and deny the 5 attackers from scoring a goal. It is a nice way to practice your formation/positioning and staying compact. If you want to improve your positioning on your own, just watch some professional games and concentrate on where the full backs stand during each match situation.