Remove your stock from the mechanical portion of the airgun so you don't damage the airtube. Place rail and center on stock. Use a felt tip marker to mark the holes once you align with a ruler. You can even tape the rail with blue tape to the stock and look at it from different angles to make sure its centered. Measure twice or 3 times to make sure the 2 screws will be where you want and use small drill as a pilot hole 1st before you use your regular drill bit.
With screws, there are many options out there. I personally like to use SS screws with T-nuts on the inside of the stock as they pull themselves in, you may need to use a Dremel to cut into the inside of the stock for the T-nut to fit against the stock. I went to Ace hardware and purchased my SS screws from them.
.