Step 1: Materials and measures
2. Cord of some kind for shoulderstraps. 3 meters or more depending on your size and bag size (little bag - longer shoulderstraps). I had two 150cm shoelaces, so exactly 3 m and I consider myself little. It must go four times from the top of the backpack over your shoulder and to the bottom corner of backpack.
3. Optional: Large buttons, 4 of them. You'll see in step nr 9 what for exactly.
5. Sewing machine, scissors, measuring tape, pins, needle, large safety pin.
FINDING YOUR MEASURES:
Think of how big you want your bag to be.
Add 2 cm (or whatever you need for sewing an edge and multiply it with 2) to LEFT, RIGHT and BOTTOM of the main parts.
Now look at your fabric and and cord and think (or test) how wide should be the tube that would allow fabric to gather enough into creases on the cord. What would look and work normal for closing and wearing the bag. Found? Then add TWO TIMES THE WIDTH OF TUBE PLUS SEWING EDGE to the TOP of main part.
Note: if your sewing machine does not have ability of zig-zag or overlock you should try to use the fabric's non-furcating edge for the top edges of both main parts.
Choose your width and length for the handles. Double the width and add sewing edge to both sides. For length I can only tell that I made it 30 cm and it feels just a little bit short. Consider that attachment to the bag takes up at least 1 cm on each end or more if you want it to be more decorative. If you can zig-zag or overlock you don't need to add extra sewing edge to the length, but if your'e a perfectionist you might want to.
Actually you don't have to measure and calculate in numbers but you can just imagine and consider all the extra edges just before cutting. But check tvice before you cut.
Now try to measure the way of imagined shoulderstrap taking in consideration also the height of your bag-to-be. Or if you have lots of cord laying around you can work it out later when the bag comes to existence for exact testing.