Painter's tape it to either table tops or hardwood floor. I have two large banquet tables put together. Getting down on the floor just doesn't work for me anymore. These old knees can't manage that. I put two tables together and tape the edges down on my backing, add the backing and tape it too. Then the top and tape it as well. Pin away!
I now use the spray basting which is like the most wonderful thing ever! Spray the backing, lay it out smooth. Spray the top reverse side, of course. Place the batting down on the backing and smooth it out. Then place the top on the batting and smooth it out. ....no pins!!! and it's repositionable.