When I diaper up, I found standing and leaning against the wall the easiest way for me. I seem to get a tighter fit standing vs. laying down and can center the back better. I also always tape the top tapes first, then the bottom tapes. The top tapes point at eachother and the bottom tapes angle slightly towards the floor pulling the center up (if that makes sense).
Honestly I don't think there is a set in stone, correct way ( but there are wrong ways)! As long as you get the fit tight (but not too tight) and the diaper does not sag creating open areas (before maxed out) you are doing it right.
I diaper standing up probably 99% of the time. I lean up against the bathroom counter. This allows me to easily even out the diaper for the best fit. I tape the bottom tapes first, then the top, and then middle if the diapers have six tapes.