Stuff you will need:
*2 coordinating cotton fabrics: 50 cm each (and the usual width which usually is 140-150cm)
*a piece of soft clear plastic (like the one you use as tablecloth.) 27x15cm or depending on what size your window is.
*Basic things like sewing machine, thread, iron, scissors, ruler and some pins.
I've included seamallowences in the measurements. They don't have to be exact. I always use the edge of my foot for standard sewing and then a mark on the foot for the narrower seams. Just make sure you use the same seamallowences and you'll be fine.
0,8cm for the straps, sides and bottom of bag
0,5cm around the top edge
0,3cm around the window and reinforcements for the straps
Finished size of the bag: 36x37cm and 52cm long straps
Step 1: Sewing the straps together
2 pieces 37x40cm in black
2 pieces 37x40cm in white
2 straps 55x5cm in black
2 straps 55x5 cm in white
1 clear plastic piece about 15x27cm
Sew together the straps along the long edges, one white and one black right sides facing. Do the same with the other one. Turn them inside out and press the seams. If you want you can now add a narrow decorative seam along the long edges. I didn't on mine. Cut them to the exact same length if they aren't already and set aside.
It can sometimes be a bit hard to turn the straps if they are too narrow and long. I usually use a safetypin which I pin on one edge and then lead through the tunnel. Do little at a time making sure it doesn't bundle up too much before you continue leading the safetypin through. It also helps to moisten the strap with some water. Just be careful as the fabric will be more delicate and might break more easily..