Many already has its mold they use and are satisfied with it.
However people who are starting have some difficulty getting or make them.
I designed this simple mold whose dimensions are 300 x 90 x 100 mm that allows 12 bars of soap 130g in standard size of 90 x 60 x 25 mm. For this standard size 12 bars the soap mass required is 1560g. As has a height of 100mm, allows taller bars, limited to 85 mm.

If want to make a cutting guide for cutting the soap with a knife or blade or a stainless steel wire stretched, has a drawing of how the grooves in the mold.
If you want a smaller mold, just resize the horizontal parts, for example, for 6 bar measurements are by half.

The photo posted above is a old model, has only one difference in the side piece.

You can download pdf document on downloads page – click here