Going somewhere else where you can use big tables would be my first choice - but I have and still do sandwich quilts on my bedroom carpet. Lay your backing fabric down, wrong side up, and pin it to the carpet around the edges. Layer your batting over it and smooth out. Then lay your top down, right side up, and smooth it out. Now, using your cutting mat, slide it under the sandwich, between the backing fabric and the carpet (you may have to remove some of the pins) .Now you can pin the sandwich together, using the cutting mat as a solid surface against which you can pin. Still a lot of crawling around on your knees, but it works!