I've been messing around moding gear since I was a kid, really. Typically simple stuff, like adding compression straps to an old rucksack back in middle school. More recently I added a extension skirt to a pack, and made a throw bag for packrafting. Stuff like that.
This was the first garment I've made from scratch, and the first time I've made anything from a kit. The directions online were quite clear, though I made the collar my own way, and obviously changed a few things to allow the pullover style.
I should say that my wife sews professionally, so we have a very nice sewing machine (does something like 50+ stitches) and her as a sounding board for a few things. For instance, the combo of slick low denier nylon and fizzy Primaloft makes creep while sewing a big issue. When sewing the liner to the insulation I pinned every 1.5 inches, and this was not overkill.
I would no try this project without a machine that will do an overcasting stitch, joining the pieces in the end stages would be murder without finished edges.
Another wise thing I did was cut and sew a dummy test vest with some cheap fabric we had laying around. It allowed me to tweak the fit (slimming the torso, mostly) when I cut the pieces for the actual vest.
I also cut all the pieces separately. Laying the liner and outer layers and trying to cut two at once would be a recipe for pieces that don't match.
The whole deal, from cutting the test pattern to being totally done, was about 7 hours of work.
I'm psyched to have a better fitting version of something like a Patagonia Puff vest, for less than half the money, and in the color I want.
Wore it to the bakery to get cake for lunch in the rain, and it was nice and warm!