Instrument? Doesn’t matter, but wider string spacing helps to start.
Strings? Doesn’t matter, although some prefer a lighter gauge with a low action.
I’d go with a clean tone, certainly to start with.
Happy, smiley, mid-scoop EQ, to start with.
Comp helps, if you have it, but not super important at the start, fast attack, and release to taste.
I feel it comes from the wrist, but your forearm can’t help but follow and keep the thumb loose (keep it all loose in fact).
The rest is all what you fancy/feel comfy. For me, again, FOR ME, I like to slap against the last couple of frets and pop with my index. The action feels like it’s straight down, 90° to the neck, but given I use a rotation of my wrist/forearm, it’s probably more down & across the string towards the floor. In terms of up and down (double thumbing), I’d stick to down strokes to start with, as double is probably more an advanced technique, but try it if you want, it may feel easy and natural, especially if you’ve used a pick and use alternate up and down picking.
As with any new technique, start slow and easy, maybe just thumb with no pop for a while.
This is all what I would do, again, WHAT I WOULD DO, there are no hard and fast rules as this is still, in the grand scheme of things, a relatively new technique and there is certainly no right or wrong way to do it. However, watch out for bruising on the side of your thumb (just look at how much tape Mark King puts on his thumb).