OK, an off-the-cuff tute...
I use all kinds of fabrics - curtains work well because they have a bit of stiffness to them, but duvet covers can be cool too, though more floppy.
The basic shape is a rectangle, you choose the size to suit the bag you want and the fabric you have. Just don't make it so big that it will drag on the floor (I did that once) or so small that you can't fit anything through the opening (I did that too
). I try to cut them in a way that suits the pattern on the fabric - the oriental red design wastes a lot of fabric in order to centralise the design.
I don't line the bags and they don't have any way of closing them (I hope to learn that soon!). So just pin them right sides together and sew 3 sides. You cut 2 long rectangles for handles - again, judge the length yourself. I like to carry mine in my hand rather than on my shoulder, so I don't want the handles too big.
I fold one third of the strap in to the middle, then fold the other third in half and then over - so there is no raw edge showing - and stitch along the join. Then place the strap on the open edge of your bag and position it so it will be centred and so that it is a comfortable length. Turn in bag edge over twice so that the strap is double caught under for strength. Pin both strap points on each side of the bag (ie each strap twice - each end of the strap) and then turn the rest of the bag edge to match all round. Sew all round. NB The fabric where the straps are will be bulky so slow down here.
Last of all, fold the straps so they lie pointing upwards and stitch a box and an X at the strap anchor points.
I hope that makes sense - please ask if I need to make anything clearer!