I used to have trouble with transmission pans. After I found we had an inch pound wrench I used the spec, typically 12 in lbs, found it took several rounds to get them all tight at once, no more leaks.
Same here, oil pans and valve covers too.
#1 important thing for pans and covers is clean and check them with a straight edge. Too many gorillas work on the car before me and think tighten it until it bends the wrench and the pan.
For trans/oil/valve covers I do not use anything bigger than a 1/4" drive ratchet. Same thing with plastic intakes.
I do follow the gasket manufacturers recommendation about sealant but if it doesn't have specs I do this.
Trans pans I oil both sides of the gasket and snug repeatedly in a crisscross pattern until it feels good or the gasket starts to just barely squeeze out.
Oil pan I spread a thin thin smear of sealant on both sides of the gasket and snug repeatedly in a crisscross pattern until it feels good or the gasket starts to just barely squeeze out.
Valve covers, I use sealant on the valve cover side of the gasket and oil on the bottom of the gasket then snug then repeatedly in a crisscross pattern until it feels good or the gasket starts to just barely squeeze out.
If I have a leak I snug a little more but that doesn't happen often and it is usually when a center bolt valve cover has been bent in the middle.
The most important thing is to do what works best for you.
