After pressing as well as I can, I fold in half once width-wise and then once length-wise. This little bit of folding helps to mark the centers for me.
Then I use a long rectangular table -- find one at a church fellowship hall, community center, library conference room; center the layers on the end of the table one at a time (stretching from center to one end of the top), clamping them down individually with big "bulldog" clamps; stretching/smoothing slightly as you go (this helps remove the 'wrinkles' that have occurred from handling the top after pressing).
Allow the excess to hang over the sides and (hopefully very little) on the ends. I hand baste because I mostly find this keeps my really big quilt's layers flatter than I can even with spray basting.
When I have well basted from the center toward one end, I re-position the quilt so I can get from the center to the other end. Keep moving, smoothing, clamping, and basting until you have the whole quilt done.
Basting a quilt is not always a fast process, it can be quite a boring chore, but rushing through it will definitely affect the outcome of the layers of your finished project.
Then, there is the board method taught by Sharon Schamber (sp) on you tube. I have used this and did well with it. Because it was not as familiar as the table clamping, hand-basting method I've used for years, I'll probably go back to my method next time.
Jan in VA