are the most difficult part of DIY stuff. I just built a 3 person tarptent (full netting, front/back beaks, sewn in floor) which weighs 25oz (before seam sealing).
The hardest part was determining the size/shape of the beaks and the wacky front netting door (single piece, no zipper, but still rolls away). I had to whip out a bit of trig ("Law of Cosines" , etc.) to get it done.
The next most difficult part was cutting the large pieces of material. The thin materials like to bunch up, strech a bit, wrinkle, etc. So, making cuts on loose fabric is difficult. I ended up taping my sheets of silnylon to the kitchen, or garage, floor with masking tape and then placing duct tape over the masking tape. This would allow me to hold the fabric still so I could pull out the wrinkles and keep the sides squared up.
Relateded to cutting, is pattern layout (not sure of the term). Trying to determine the best way to cut the material so you have the least amount of wasted material. Again, a bit more trig was needed.
The actual sewing part is very, very easy. If someone would have designed and cut all of my material for me, I could sew another tent in a coupe of hours. The design and cutting easily took 40+ hours.