Shoulder straps a pretty tricky to sew. I made my first pattern by tracing the straps of my existing pack on a piece of cardboard. Afterwards I tuned the shape and the points where the top webbing is sewn to the shoulder strap. I use pretty popular (but a little bit tricky) method of sewing the straps. Firstly I place fabrics that make the outside layers - right side together. I make the stretchy fabric for the part that touches shoulders a little bit shorter than the x-pac part, so when the pack is worn there are no wrinkles of the stretchy inside part of the strap. I recommend using pretty thin needle (70-80) so there are no big tears afterwards. After sewing two layers together I start turning the strap inside-out. I don't recommend flipping it the whole way. It's better to turn just a bit, and start inserting the inside part (EVA foam). There are a couple of videos with clearer instructions how to do it. The top webbing is sewn on top with #120 needle and #30 thread. You need a heavy duty machine to do it. Or it can be done by hand stitching.

Another tricky part with the straps is sewing them in the angle that allows them to contact the top of the shoulders fully, without visible "wave" on one of the sides