When I block a small quilt with steam, I just (stab) pin it to my ironing surface (you could do it on a carpet if it's too big for your ironing surface) making sure that all the edges are straight and the corners are square, easing and stretching as necessary to make the edges right. Then I use an iron with a good "shot of steam" to generously steam the quilt, especially the areas that need easing or stretching. I never touch the iron to the quilt, but let the steam do the work.
I leave the pins in place until the quilt is completely cool and dry.
If it's a big quilt, I will pin it to the carpet in my living room on top of a sheet and leave it overnight. (For this I just use water, not necessarily steam.)
Near the end of this video you can see how Alex pins a quilt to her carpet for basting.
http://www.hgtv.com/video/alex-andersons-quilt-studio-video/index.html
The same T pins work for blocking!
Although Sharon is talking about blocks, and not finished quilts, you might find these videos helpful too:
http://www.youtube.com/watch?v=i6aplw_tVZc
http://www.youtube.com/watch?v=lIjZqABo2NY
http://www.youtube.com/watch?v=sQkF02vpVuw