Just add 1/4" to each side (1/2" total for each direction l & w) of whatever size you are starting with (for the seam allowance). For example, if you want a finished 6" square, you would need a 6 1/2" square to start with. Clear as mud?
Hope this helps. This seam allowance is based on a 1/4" sewing allowance. If you use more or less you would add according to your own sewing preferences.