Typically you use as many layers of muslin as necessary for the darker color to not show through. An easy way to do this is to fuse layers using Misty Fuse, which will not add stiffness or bulk. The way I would do it is to fuse a large square of muslin to another large square of muslin, then place over the dark color to see if it's going to be enough. When satisfied that there is no show-through, I would cut out my applique from the layered muslin.
Another option is to fuse muslin to a thin layer of batting. This can add a slightly trapunto effect to the applique.
Last edited by Prism99; 02-04-2012 at 09:45 PM.