The first thing that you need to do is to get the right socks. Keep in mind that the socks that are worn with the baseball uniform rise up to the knee. More often than naught, you will find it a bit hard to find socks that rise up to the knee in sporting goods shop. You may have to visit a few shops before you find them. In case you are unable to find the appropriate high socks, go for the soccer socks instead, as they are typically long in order to accommodate the shin guards. Do not make any compromise on the length of the socks, as they are an essential.
Before you put on your pants, put on your long socks. You need to wear the socks before putting on the pants in order to get the right fold just below the knee. Otherwise, you will find yourself spending a fair amount of time making adjustments to the height of the fold. After you put on your socks, pull them up all the way to your knee.
Now that you have put on the socks, you can proceed with putting on the baseball pants. The bottom part of the baseball pants typically reach down to the ankles. However, you need to fold them up to bring them to the length that you want, which will typically be up to the shin. Do this by give the pants an inside out fold. Make sure the folds are neat so that the bottom of the pants does not fall down to the ankles again while you are playing the game.
Pull the socks over the bottom of the folded pants, so that you can no longer see the point where the baseball pants end.