I am about ready to plant my NPT and i was wondering should I cap the soil before I plant or plant first then cap?
I capped first then planted with about 3-5 inches of water above the cap. I hold the plants by the base and wiggle them through the sand into the soil unless, of course, the base is to remain exposed, I'll wiggle just the roots in with my fingertips. A small amount of soil may
float up while doing this, but it will either settle back to the bottom or float to the top of the tank. If it floats, skim it out with your net, and if it sinks, you can remove it using a gravel vac in one hand while stirring up the unwanted debris with the other hand as to keep the sand cap in place (do not try to vacuum it directly off the top of the sand, because you'll take the sand too).
I chose to leave the sunk debris on top of my sand for now (I set this tank up Saturday) - it doesn't bother anything, but during my next water change, I plan on using the method I just described to get my cap looking nice and clean. OFL suggests only vacuuming one part of the cap at a time (mentally divide the tank floor in sections according to your tank size) so the mulm (fish poo/plant debris/uneaten food) has a chance to provide nutrients for the soil.
If you're going for a super clean look for your cap, you can plant before you cap, but be careful not to allow the weight of the plants leaves to strain the plants too much, and be careful with plants whose bases are to remain exposed to the water - you should plant these after capping. Having the extra 3-5 inches of water in the tank will prevent the plants from weighing themselves down.