This is what the robe looked liked before I started cutting:
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I made a rough pattern by tracing a pair of his pants that fit onto some brown paper. Then I just laid that onto the folded robe and cut away.
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By the way, that is a little trick I learned from my seamstress friend, use canned veggies or soup to hold the pattern in place if you don't have pattern weights. They're so handy, especially when using that brown paper because it is hard to pin through.
I cut the pattern out on the bottom of the robe as it was folded so I ended up only having to sew the inseam and crotch seams (no side seam or hemming needed). Then I made a casing for the elastic. Easy peasy.
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~Megan