Go to Thru-Hiker.com and there is a great article there on making stuff sacks. That's how I learned. You end up having a square bottom.....on the stuff sack.

I make my own seam sealer with silicone caulk and white gas.

Sewing silnylon takes some practice. You can either try pinning it and taking them out as you go or do a rolled seam and keep the tension on the fabric and kinda guide it through the foot. I pin the first part and do the rest free hand when making tarps. When making clothing, I pin everything.

Having used a bandsaw and learning to cut all different kinds of wood freehand really helped with sewing.

And oh, by the way, a sewing machine is just an indoor power tool!