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.
Step 2: Cut
2 pieces main part (front and back of the bag)
2 pieces handle parts
If you can overlock then do top edges of main parts and both narrow ends of each handle piece. If you can zig-zag then wait. If you can't any of these then I hope you noticed in previous step what to do.
Step 3: Mark and sew main parts
2. Remember the width of the tube you added to the top? Measure the double height plus sewing edge and mark width pin or chalk.
3. Pin the rest of the edges (except top and what's on the top side of these markers) .
4. Sew the edges from one marker to another once (right side out and ugly edges also? - YES, that's right).
Step 4: Continue to sew main parts.
And sew the same range as before - from marker to marker - but with a little wider extra sewing edge. Definitely wider than the ugly edge you made before on right side. (This method hides the ugly edges on both sides.)
Begin with double or triple - going back and forth near the marker - so it couldn't go loose. Better sew it all double.
Between the just made sewings and top edges there is need for some cutting. Look at the pictures 1 and 2 below.
If you planned to overlock fabric edges now is right time.
When that's done turn the bag right side out. Where left and right sewings end at the top, there continues the unsewn corner which now should fold naturally and obviously. Picture nr 3. Fold it and sew it zig-zag or staight. All 4 corners.
Now fold double both top edges so that they would reach the point where left and right sewings start. Picture nr 5. Attach with pins. Attach first both ends then center and then inbetween, so you get everything even.
Sew so that you get two tubes with open ends. Sew double and make both ends especially strong by going back and forth few more times. If you planned for zig-zag now you can do it.
Step 6: Handles
Step 7: Attaching handles
Note: Handles must be attached lower than the tube for strings.
Sew them on place. Just one line might not be enough, use some rectangle or whatever pattern you like.
Step 8: Attaching shoulderstrings
Use big safety-pin to push the longer end of the string through one part of the tube.
Put the bag on your back and strings over your shoulders and find out where to tie the other corner.
After making the other knot similarly push one end of the string through the other tube and tie the ends together. IMPORTANT: check that both strings are even in length. Adjust if needed. If you estimated right, the knot should align into the tube and be hidden. If not, adjust the knots .Trim excess lenght.