1) Start with the pattern.
(If you need more help with the pattern making, go to:

and: http://www.creativecutie.com/2010/01/patterns-part-2.html)

This is a 3 parts pattern. Cut the pattern according to the main measurements and as you see in the picture (this is folded in half):

And draw a line like you see in the picture above.

This is how it looks like when whole and laid out:

Keep it folded, and cut along the line you drew:

You will get this:

You can throw out one of the side parts, you will only need one of them and the middle part. These are the main body pieces. There are 2 pieces. But you need 3 - the 3rd will be the flap. This is next. Cut:

This is it for the pattern!
Now let's start with the bag itself!

2) Take the side pattern piece, and cut out 4 pieces for the sides from the main fabric – 2 for the left and 2 for the right.
Note: you can use the same pattern piece, just use each side of the pattern for each side.

3) Also, cut from another fabric 2 pieces of the middle part:

4) Cut out the flap pieces – 1 from the main fabric, and 1 from the lining fabric.
Note: I used the same fabric for both the lining and the middle pieces. You don't have to do the same as me!

5) Attach one half of the magnetic snap to the lining piece, 5cm from the bottom:

Optional: put on the interfacing.

6) Place the flap pieces right sides together and pin. Stitch all around, and leave a gap at the top part:

7) Turn inside out, press, close the gap (optional because it's not critical) and stitch it all the way around nicely. This is the flap from both views:

Done with the flap!

Optional: I used piping for this bag, but you don't have to. If you do, do it now, before attaching the pieces together:

8) Sew the pieces together - right sides together: the side pieces to the middle pieces:

Unfold and press nicely:

9) Put one of the full pieces you've just sewn on the lining fabric, mark around and cut. You'll need to do it twice:

10) Attach the other half of the snap to the bag's body, 6-7cm from the top:

11) Add interfacing.
Note: it can also be done before the previous step, depends on the interfacing you're using.

12) Make the strap. If you're using rings like in mine, make 2 more straps besides the main strap, shorter ones.
Note: I made the strap with 2 fabrics – one for each side of it, so keep in mind the seam allowance however you decide to make the strap!
First cut the 2 fabrics 6cm wide and decide on the length:

Turn inside out through the gap, iron flat, and sew lengthwise, closing the gap by doing it:

Now the other 2 smaller parts. They will need to be about 4cm wide and shorter than the long one. Sew them like this:

Turn inside out and do the same as you did with the other strap piece.

Put together the whole strap unit like this, and sew:

13) Sew both exterior pieces together, right sides together, like this:

 

Note: notice that I didn't sew the triangle holes at the bottom!

14) Sew both lining pieces together as well, the same way, only this time leave a gap at the bottom.

15) Make boxing with the triangle holes at the bottom sides – bring the triangle's sides together, and sew both sides like this:

16) Pin the flap to the exterior, its face down and the magnetic snap closed (this way it'll be easier to work it). You need to sew it to the exterior's side where the other half of the snap is not on, like this:

 

17) Pin strap to the exterior's sides, face down, and sew:

18) Turn the lining inside out so that its right side will be facing outward.
Insert the lining into the exterior and align both to each other. Make sure the strap's ends and the flap are tucked in.
Now sew all around the top:

19) Reach into the lining and pull everything through the gap in the lining, turning it all inside out.

20) Sew the gap in the lining closed:

21) Tuck the lining into the exterior, iron the edges a bit and you can topstitch all around.

And you get this: