Question about stapling 1/4" plywood underlayment?

I'm about to attach my underlay to my 3/4" Advantech. I am unsure about the staples I need to use, or can use. I have an electric staple gun that can shoot 1/2" long 3/8" wide staples. Are these acceptable? Also, on the seams I see a few different recommended gaps sizes. The most common one I see is 1/4" from the wall, 1/8" for the inner seams. Other gap measurement recommendations are 1/8" from the wall and 1/32" in the inner seams. Any advice would be great. By all means I am not an experienced carpenter - I'm just without a lot of money and need my floor down. I'm building a cabin and I barely have enough money to do it myself. I just want to make sure to put it down proper so I don't have to come back in a year and rip it out or reattach it.