This is an upgraded version of https://www.thingiverse.com/thing:3157323, with different anchors to fix the strings and a patterned sole and insole.
You can change the thickness and modify the shape to match exactly your foot and preferences, change the pattern of the sole and the pattern of the insole in contact with your feet.
First you have to put a sheet of paper under your foot and with a pencil draw the contour of it and mark a few points.
You can select to show the points as in the picture attached, just set the Edit_mode parameter to 1 and you will have an idea of the points you have to measure.
Draw a horizontal line and a vertical one starting from P1. Now draw 3 horizontal lines (it is not important the vertical distance between them, just space them roughly equal).
You have now your reference points and you can fill in the parameters of the customizer, either online or, if you downloaded the file in Openscad customizer, just change them in the file itself.
All the points are shown in the rendering window, with each ones coordinates. In the parameters you have the contour fixed points section, fill it in and after you need to adjust the Control Point, just by moving them around a bit. To have a perfect curve joint, the control lines arriving and leaving the contour fixed points should be as straight as possible.
When that's done, you have to arrange the rotation of the anchor points for the strings and you are ready to add the strings. You can change the diameter of the holes for the strings, in the actual parameters it is good for 1.75mm filament as I will use the same flexible filament I use for printing the sole.
The strings will be inserted in the holes of the anchor points and fixes with a drop of CA glue or just fusing them with a lighter at the bottom of the sole. You can place the strings in different ways and add two more anchor points to add 4 more strings.
Done, you've got a tailor made flip flop !!!
Print with flexible filament.
In the example the sole is 6mm thick, and I suggest to print with an infill based on how you would like the sole softness. infill around 10% should give you a very soft sole, 20% medium and if you like it pretty solid, 30% should be enough.
Due to some memory limits for the online customizer, the sole pattern settings may cause the rendering engine to fail showing the correct object. To avoid that, reduce the density of the patterns till you get a proper result.
To set smaller patterns using Openscad you need to "turn off rendering at" to an higher value.