I start with the paper i want to use, a pencil and a divider in the correct size (in this case one of the original ones from my Calipso Filofax).
Trace around the divider and inside the punched holes.
Notice in the above picture where the original tab was.
I'm making top dividers for these ones so i'll just continue the cut line all the way.
Now for the next step. Making the divider tabs. You'll need the pencil, the divider you're working on and a tab (I use the dividers from the pocket to make the tabs since i want my tabs a little bit smaller than the original A5 tabs).
Place the tab where you'd like it and trace it with the pencil.
Now you'll need a paper cutter and/or scissors.
Cut around the tabs cutlines and along the sides til you get your divider.
One almost finished divider with top tab.