I recently did this very thing. I studied everything I could find on YouTube, Google and on here. Long story short ... it was VERY EASY. No problems at all.
Mesa Tactical Rem 870 Receiver Rail, 5 1/2"
Link $29.99
Hanson Carbon Steel Bottom Tap 6-48 Thread
Link $2.29
Baker Drill Bit Jobber Length Titanium Nitride (TiN) Coated #31
Link $3.49
Harbor Freight Transfer Punch Set $11.99+
I simply measured center, drew a line using a pencil, positioned the rail, center punched one hole at a time, used a cheapo drill press to drill 1 hole at a time, a cheapo drill press vice to hold the receiver, lube while drill & threading and just took it easy with the thread tap. It took me maybe 1 hr or less to do the entire job and I was pressed for time at the time I did it. It is very easy to drill & tap threads. I believe it is most safe, when tapping threads, to twist the tap maybe 1/2 turn right then 1/2 turn left just to keep the tap clean and to not gall the threads. It really is a simple process, but I TOTALLY understand the stress about ruining the receiver!
Once all threads were tapped, I installed the rail using Rocksettt threadlocker to make damn sure the threads lock into the receiver.