A few things to add to the good information above. Do not use steel wool if you are using a water-based poly. Any bits of steel you don't pickup will rust and cause problems with the finish. Be careful not to round over the edges of the table with the sandpaper. By the third coat, you should have a nearly perfect surface assuming a fairly dust-free environment. Last, if you are using a regular poly, you can do a light, final buff with 0000 steel wool, perhaps with some wax (tinted or not) to flatten the final coat and add some lustre.