And depending on what you install on the firewall, once finished there might be other unused pre-punched holes and you might need to open up new ones.
Fill them up with whatever appropriate (i.e. rivets, screws, bolts, stainless plugs, washers, etc.) and seal with Fire Barrier.
To create holes, there are threads here you can find with lots of good suggestions, in essence it's tough to drill stainless, use support on the other side, super slow speed, pressure with sharp bits and lube. Best is to punch through for larger holes.
2023 =VAF= paid
RV-8 SB #83274
Grove Airfoiled Gear, Wheels & Brakes
VPX-Pro, EFII System 32, EarthX batteries
Aerosport IO-375, MTV-9-B/183
C-GEMQ, RV-8, "Dragonfly", flying
C-GNFJ, C72R Cutlass, flying