I drilled & threaded a Remington 870 and it wasn't a big deal to do. I called around and people wanted too much money to do it for me. Just get the appropriate Picatinny rail, the proper thread tap for the screws and use the right size drill bit.
1. I measured and scribed a pencil line to mark the center of the receiver.
2. I centered the Picatinny rail over the scribed line and marked one screw location using a center punch.
3. I drilled & threaded that hole, installed the Picatinny with the scew. I lined it up and punched another screw location.
4. I removed the Picatinny rail, drilled & threaded that hole and repeated the process until all locations were drilled & threaded.
It's nice if you can use a drill press, even a cheap one along with a vise. It can be done with just a hand held drill, but a drill press & vise ensures the holes are drilled as plumb as can be. Take your time drilling and use lube. Same with threading, use lube and go slow. Once I got the thread tap started, I would thread the hole a little ways, reverse the thread tap up and screw it back in again. It went perfectly.