To get the pattern, I laid out a pair of their footie jammies on a large piece of paper (I used the jammies because I knew I couldn't get them to lay still) and traced around it leaving a large amount of kicking room. The back has flaps on either side to wrap over their body and is a single layer of fleece. The foot sack is made of two layers of fleece and comes up to about their armpits. To get the measurements for the straps, I just placed it on the stroller seat and felt where the straps went and cut slits. Later I think I will reinforce the slits by zig zagging around them with my sewing machine.
Doesn't he look cozy? Now I just have to make some warm booties for their feet and some mittens that will actually stay on.