Floor ubderlayment

My family and I will be having vinyl sheet flooring put in by an installer but we will put the 1/4 inch underlayment down ourselves. I do not know which would method of installation would be better. Should I use staples with an air gun, or ring shank nails, or should I screw the underlayment to the 3/4 inch sub-floor? What spacing along the edges of seams, and in the rest of the sheet should I use? Should I butt the sheets together leaving a small gap around the wall edges? Should I leave a small gap between the sheets and use a leveling compound? Thanks for any advice you can give me. Larry