You can set a solid sim-color map right in the first clothing creator tab. Pure red is rigid, black is loose, all other inbetween is ... inbetween. Try to set 60-70% of red on the slider and watch the effect.
Though, for good sim effects, you would like to paint a sim-map.
For this, I would recommend a 2d painting tool like Photoshop, or one of its free competitors.
It would be good to have one that can uses layers, but even the primitive Windows paint will do it.
As you asked for it, her is my workflow:
-Go to the different material-tabs in the clothing creator, navigate to the textures sub-tab and click on the top left to generate a sim map template. Instead of this, you can just copy one of the clothes' textures to paint over it, too.
-Open this sim template in your painting software.
-Look at the UV-layout and try to understand the different clothing parts. Your texturing-experiences will help a lot.
-Create a new layer. With pure red selected (RGB 255,0,0) use a brush and paint on the seams that should stay rigid, like the waist region of trousers, for instance.
-If the UV-layout shows some loose parts, like buttons, pockets or a seperate waist band for trousers, paint them red, too, or they will fall apart. Some try-and error will help.
-Now generate an other layer and use gradiation fill on it from red to black. Or just fill the complete layer with pure red. If you control the transparency of this layer, you can control the rigidity, too. 75% visibility of red over a black background is the same like 75% pure red.
-If you want the sim map to have regions without any rigidity at all, just erase those areas from that map, or use the gradiation fill, if possible. In my experience, it is good to left a bit of rigidity and do the fine tuning in the clothing's physics settings.
-At a last step, generate a new layer, fill ot with pure black (RGB 0,0,0) and put it in the background.
It is true, you can theoretically left the black and blue sim-template visible, cause only red color matters, but a pure black background looks much cleaner and, for me an important cause, it saves some KByte of disk space!!
-The layer ranking should now be: upper most layer is the one with the painted pure red seams, beneath this comes the layer(s) with the gradient or transparent fill (mind that two transparent layers with 50% red will add to ~100% red, again), and the last layer is the pure black.
-Now save it as JPG with low quality settings. This realy does not make any difference but saves disk space. Keep the layered composition open or save it for later as a layered format like .psd, for instance.
-In the clothes creator material, there is a small entry at the very bottom of the texture sub-tab.
Add your new sim-map there, go to the main creation tab and activate "use different maps for materials" (or how it reads).
-Try out your new sim-map. Most likely you will feel the need to do some changes.
-Now the layer-workflow will show it's strengths: correct the transparency settings of the middle layer(s) to correct the rigidity, or add a new layer with some regions of more or less red at different parts. Having those layers will save you a lot of time with that later changes and adjusting.
-Save and try again...
As said above, you can do this without layers and with the Windows painter, too. But much less comfortable.
