Practice in an open parking lot. Try to stay in lanes and back into "spots." Pull about 10' past where you want to back into and then cut the wheel all the way to the opposite side. Like youre backing the truck itself into the opposite side. This will "jack" the trailer. Then watch the mirrors and start to "chase" the trailer.
A little practice and you will have it down. I got good at backing up trailers years ago with my boat. My TT took a little practice and the 5er a little more. Of the 3 the 5er is the easiest for me. I just think of it like jack and chase. You will catch on quick. And USE your mirrors. If you cannot see something GET OUT AND LOOK.