Basting spray is fine. Many people do use it; I don't because I'm allergic to the propellant. I would recommend 505 Basting Spray if you're using that as it's water soluble & repositionable. I wouldn't bother with the basting guns.
For Sharon Schamber's method, the 2x4's need to be wider than your backing fabric. Your backing fabric should be at least 2" wider & longer than your top -- 1" all the way around; I try to make mine a minimum of 4" wider & longer -- this is because some of the backing will be taken up in the quilting process. When you start, make sure you have everything centered with the proper margins at the edges. If you're using boards on both the top & the bottom, I would pre-mark the center of the top & backing so you can match them up with a straight pin before starting. If you're just using one board and it's a large quilt, you'll need to work from the top to the bottom (or bottom to top), working from the middle out on each "row".
I don't know what the sheet idea is, unless you mean that you're backing your quilt with a sheet. If so, make sure you gently smooth the sheet-backing with flat hands & tape to the floor with masking tape (or if it's carpet, secure the edges with T-pins) before you lay the batting & quilt top on top. For that method, because the entire quilt sandwich is laid out at once, you do want to work in quadrants from the center out, and ideally work with diagonal quadrants to help keep it centered (i.e., pin the upper right quadrant, then the lower left; then the upper left quadrant & finally the lower right). Working in that method helps move the top evenly across the backing to ensure it's centered.
Here are a couple videos:
Clipping to edge of table:
https://www.youtube.com/watch?v=BT5ywYhlKe4
Pin basting on the floor (surface large enough to lay out entire quilt):
https://www.youtube.com/watch?v=3wRxkSWax9Q
Really good Sharon Schamber video:
https://www.youtube.com/watch?v=HyaLsMafElo
Spray Basting:
https://www.youtube.com/watch?v=MSeWBOnbC90