Try this: While standing, hold the diaper behind yourself by the top tapes (or the only tapes if it's a one-tape-per-side diaper). Raise or lower the diaper until the tapes are at the level of your beltline, then pull them forward around yourself until the back of the diaper feels comfortably snug through the small of your back and above your hips--just like a belt would be. At this point, note the approximate separation of the tapes. (The front of the diaper should be hanging loose behind you throughout this exercise.)
Next, lay the diaper out flat on your bed or a table and, with a magic marker, draw two parallel lines down the front of the diaper, with a separation similar to what you observed with the tapes.
Finally, proceed to put the diaper on. Position the back so that the top (or only) tapes are at your belt line just like you did earlier. Pull the front of the diaper up between your legs until it's comfortably snug through your crotch. The front may be a bit high on your tummy at this point, and that's fine. Pull the tapes forward until they reach the lines you drew, and stick them down. You may find that angling the tapes downward a bit helps with support. If you have another pair of tapes below, you'll need to experiment a bit with placement. The best placement for those will depend a bit on your body shape. In most cases, I end up angling the top tapes down and the bottom tapes up so that they're almost on top of each other.
If the front of the diaper rises well above your belly button, there are a few things to try. The first is to simply accept it, as babies often wear their diapers this way--so hey! Authenticity! The next would be to tuck the excess in, possibly over your penis if you're a male who likes to "point up". If the excess is extreme, like halfway to your nipples or something, you probably have the wrong size of diaper, but you might also try repeating the above process starting with the bottom tapes instead. That will distribute the excess evenly in front and in back. In the end, you just want to make sure that one pair of tapes is tensioning the diaper like a belt. When I do this, I've never found it possible to slide my diaper up and down, but then I'm definitely not a stick!
Good luck! It does take practice.