Underlayment comes in several types of material but the most common are quarter inch thick luann plywood for carpet and quarter inch thick tempered masonite fro tile. Many carpet manufacturers make their own underlayment that has grid line markings for the locations of the fastening nails.
You will see that in order to get a warranty for your finished floor the plywood sheets must be nailed at four inch centers along the edges of the sheets and six inch centers in the field. That’s a lot of nails. Renting an air powered nailer for a day may be well worth the expense.
Lay out the floor of your room to achieve the maximum coverage of each sheet without cutting them. Check the room for square. You may have to cut the sheets for an entire side of a room to keep the rest of the sheets square but by dong so, installation of the remaining sheets will go much faster.
Most flooring contractors use ring shank six penny nails to fasten the plywood. These type nails will not pop or back out over time. They can also be tough to pull back out if you bend one.
Start by using a full sheet at one end of the room proceeding along one side until you get to the cut piece at the other end. Whatever length cut piece you end with, start the next row with a piece that size.
Then start using full sheets again. This way the panels short seams will not line up across the room. Staggering the seams makes for a better and stronger floor. If using a power nail gun, check to assure that the nailer is driving the nails just below the surface of the plywood. Never allow the nails to be flush or above the surface.
They will quickly wear through the finished flooring. Once all the plywood is in place, a careful sweeping and then a very good vacuuming is in order to remove all traces of sawdust, splinters or other debris. Anything left behind will quickly show up in the flooring adhesive making a real mess.
Once the floor area is clean, flashing compound is used on all the plywood seams to create a smooth level transition between sheets and prevent any telegraphing of plywood seams up into the finished vinyl tile.
Carpet of course is a little more forgiving in that when a padding is used and carpet placed on top, this thick layer of finished material conceals a great many sins below. You still want to provide as smooth an underlayment as you can to provide the nicest finished product.