I started by making my own pattern. It helps to do some research in advance. Try simply googling leather purse or look on Etsy, they have some beautiful stuff on there.

I love drafting with pencil and paper, but you could make your own patterns with softwares like Illustrator.

After finishing drafting, I glued it to some sturdy paper/cardboard and cut it out afterwards. Next up, I put it together to double check that everything would fit nicely and look the way I imagined.

The way I designed the inner pockets is that they're held by the same rivets as the handles. The handles themselves consist of two pieces - short and long, for a lack of better description. The short piece is directly riveted to the actual purse and the long, loose handle is connected to the short piece with a D-ring.

The way I determined where the short piece of the handles are located on the purse was by imagining where the closure flap would be once it folded over. Now I had to find out the length for the long pieces. I laid down a measuring tape and measured the distance from the D-ring when attached to the short piece and the top of the purse which came out to be ~10cm. Then I added 4cm because of the fold over around the D-ring, plus the actual length of the long handle which is 45cm. It all totaled a length of 74cm. Does that make sense? :)

Next, I created the inner pockets now that I knew where they would be attached. Nothing special to report here, the picture shows everything. ;)

For the shoulder strap I just measured the length of my current one, which I felt most comfortable with. I made it non-adjustable, just because I can't remember the last time I changed it, but that's a personal preference.