I work at an upholstery shop and do at least a few headliners a month. You HAVE to take off the old liner. Suede is much harder to work with than regular headliner material because it doesn't like to stretch. Make sure you have a wide enough piece after accounting for all the curves in the headliner. Covering mine was a pain in the ass. We use Stay-put contact cement. It comes in 5 gallon cans but I'm not sure if you can get it in aerosol or not. IF not I'd go with 3M Super 77 and you're going to need a lot of it. You need to make sure both surfaces are well covered before you stick them together. You also want to keep as much tension on the material before you stick it down. Cut an extra piece of the suede about 6"x12", coat the back with glue and fold it in half. Use that pad to brush the headliner down as you go. If you use your fingers you'll compress the foam on the back of the suede and leave noticeable lines in it.
Coincidentally I just did a black suede headliner and a fiber optic starlight ceiling in my BMW last weekend. It looks awesome but damn it took a long time and that was in a car with a headliner less than half the size of your truck. I used 197 points of light, 40 or so of which were in the sunroof slider. It took me 13 hours total including recovering the headliner. If you have any questions on it shoot me a pm.