Rune,

For installing the landing gear, it depends to the type of fuselage you have.
For composite fuses that has a flat bottom and a belly pan, it is as easy as 1-2-3.
Glue a 1/4 air plywood from the inside of the fuselage using epoxy mixed with micro balloons.
I laminate CF cloth to the both faces of the plywood. I recommend glueing plywood reinforcement at the sides of the fuselage. Not required for good quality Carbon-Kevlar fuses like PL Lazulite, ZN Enigma (NBE Mantis Ex as well)
Drill and install blind nuts at the fixing locations. 3 x 6-32 socket head screws recommended for each LG leg.

Hope these help!