|Cut your pattern pieces from muslin and rotary cut patchwork squares. Use your muslin piece to estimate the number of squares needed, working one row of patchwork at a time.
I strongly recommend serging (or at least overlocking) all seams in the patchwork AND pulling on the piece periodically to make sure it is secure.
Once you have a piece of patchwork that will cover the muslin, use the muslin to cut out the pattern piece in the patchworked fabric.
Construct the garment using the patchwork and muslin layered together.