From past experience from fitting cages in other cars. Build the cage up in the car before you drill or bolt anything in - this way you can work out placement, check everything is square and aligned, whether the dash needs cutting etc.
Also install the seats and check seat / cage clearance - sit in the seat with a helmet on and check head clearance.
If its bolt in you will need spreader plates - thick ones to put on the back side of the floor pan. All bolts need to be high tensile and as thick a shank you can get through the bolt holes. Go to engineering supplies biz for the bolt - the crap in B&Q etc wont cut it in a smash.
Once marked out and happy its spot on drill the holes, and then use paint to protect the fresh metal as a result of the drilling.
When installing the cage its a good idea to put some tension into it, which will help with chassis stiffness. Ratchet straps etc can be used to pull the front legs, main hoop legs apart very slightly to add the tension.
Also use some dum dum between the spreader plate and the floor pan to stop water getting trapped and corrosion.
Dont forget cage padding.
Take your time and get a mate to help - remember this is saftey equipment, so get it installed right and dont rush it.
Cheers
