I think I’d go for knitting rather than weaving, because it would probably be a quicker and simpler way of producing warm clothing, and I know it’s within my capacity, because I’ve already done it on a small scale.
Sheep wool would be my fibre of choice because it’s warm and easy to spin. I’d pick up the bits of fleece lying around or caught on hedges, because even if I could catch a sheep and wrestle it to the ground I couldn’t shear it. I’d get as much grease and dirt as possible off the wool by washing and beating on a stone, and when it was dry, I’d pull it over a thorny stick to straighten out the fibres. Then I’d make a drop spindle from a straight stick with a weight at one end. I’d pull out a first bit of the wool fibre and wind it onto the spindle, and then drop the spindle so that the weight made it spin round, twist the fibres into a thread, and wind the thread onto the stick.
If I hadn’t had the forethought to salvage some knitting and sewing needles before heading for the countryside, I’d have to make knitting needles from a couple of straight sticks, made as smooth as possible by rubbing with a rough stone. The diameter would have to be enough to stop them bending, but not so thick as to produce over-large stitches, and they’d have to be long enough to hold enough stitches for the garment I wanted to make. I wouldn’t try making socks, because it involves fancy stuff with four needles and turning the heel, which I’ve forgotten how to do. Instead I’d rely on making rectangular pieces of fabric that could be sewn together. If I hadn’t salvaged steel darning needles, the needle would have to be a sliver of hard wood, with a hole pierced in it — a tougher job than the knitting needles.