What you need to focus on as a fullback
Playing as a RB or LB can be one of the most difficult positions on the pitch, particularly physically. It requires constant movement up and down the sideline depending on the style of football your team plays. Some coaches prefer their fullbacks to overlap past the wingers and some are a bit more conservative and want them to stay back. Regardless, you will still be up for a lot of running week in week out.
This leads me to my first tip, you need to be super fit. Focus majority of your training on having a great aerobic base of fitness with strong legs. You'll need this base for chasing your opposing players or attacking when your team has the ball. You'll also find as a fullback that you get involved in many tackles and you'll need leg strength for this.
My second tip involves the defending side of playing as a fullback. Try as best you can to show your opposition player to the sideline. This requires you to position your body to them in such a way where the only place they can run is the sideline. This will give you more control of the situation and as long as you don't dive in, they'll have no where to go. Hold off, show them the sideline and wait for support from your teammates and you'll be a much better defender for your team.
A big part of your game will involve crossing of the ball with both feet. Ensure you give this a lot of attention in your own personal training because having this in your skillset will make you much more valuable to your team. If you can frequently cross the ball accurately into a dangerous position, your coach will want you pushing forward more often which is always exciting for any player!
My last tip is about your positioning. Always remember that you're still a defender and your primary focus is to stop your opposing winger. It's your attacking players' job to push forward and score goals, not necessarily yours. Constantly remind yourself that the first thing you should pay attention to is your defensive positioning and don't get caught out of position where your opposing winger gets in behind you.
Thanks for reading,