Well, first off, getting the new belt fed through the rail isn't too difficult, just use electrical tape to tie it to the old one and then just pull through.
Be careful to not kink, bend, or dent the new belt; those all can cause premature failure.
As for the tensioner, it has two Phillips adjustment screws on it. You will want to back those off equally perhaps 10 turns to get the belt wrapped around, then secure the ends of the belt to the head carriage, then re-tighten the belt until it is tight, but not so tight that it begins to distort (curve).
In short, the belt only needs to be tight enough to not slip on the pulleys. The tensioner pulley also needs to be evenly tightened, or the belt will start walking up and down. I usually have the carriage in the middle of the machine during this process, so that once the belt is installed and the tensioner tightened, I can move the carriage back and forth by hand to check and see if the belt walks.