What steps should i take to install my fat mat (dyamat)

[1970 TT Rustang]

I got a 70 mustang iam redoing the carpet and doing fat mat. The floors are in great shape no rust. Ive been told that por15 is great stuff to put down before hand. But i think its a little over kill, is there any other things i should consider before putting down the fat mat? Again i have no rust and solid pans. Should i keep the stock sound/ heat sheeting and lay the fatmat over it? Also how would you suggest i prep the area before laying it down?
Thanks
Rich