I find the upper tapes on diapers to generally be easy to figure out. I usually attach them an inch or so down from the top - on th padding if the diaper is a bit "long" for me, (xplus) otherwise either somewhat overlapping the padding (bambino) or maybe not on the padding at all. (mydiaper) I often do a quick measure with a tape and pretape the tops (and often the bottoms), but if you don't have the mad selection like I do here, you should be able to remember how far apart the tapes need to be to get a good fit. I don't crank the tops down very tight, I want a bit of a relaxed, comfortable fit. Try to attach the tapes equally so you get a symmetrical fit. Diapers with reliable patterns on them up on the tape area are usually pretty easy to spot the tapes on after you figure out where they go.
Lowers are a bit trickier. USUALLY they have an inch or so of space between them and the upper tapes, and usually a half inch or inch out farther. Often angled downward at a 45 degree angle or so. If you tape those down just straight in, look at your diaper after wearing for awhile, look for stretch/stress on one side of the lower tapes where they meet the back wing. If you see that on the lower part of the tape, that's because it needs to be angled outward on the bottom. Do that or you'll risk breaking a tape.
I do my fitting standing up. YMMV. Sitting changes your pelvis shape and will affect diaper fit. You can't easily get a good standing/sleeping AND sitting fit with the same taping.
And when in doubt, go large. Undersize diapers have more issues - poor drainage, popping/breaking tapes, uncomfortable, compressed jewels, etc. I'm of the opinion anyone can get a decent fit out of even a somewhat oversize diaper with proper taping. (also, before I put on this xplus last night, which are a bit to long for me and have a lot of "dead plastic" on the front end, I use a scissors to trim off 1/2 of that dead plastic - about 1.5", from the entire top end)