Hands under pants!
This is a little easier than the Futanari genitals: First place the hand or hands or tentacles or whatever in the desired position. Keep in mind the tip from above: don't be afraid of intersections!
It is also important that the hand (or whatever) does not have large differences in height. For example, if you lift individual fingers while the rest are close to the body, it can cause strange bumps
in the fabric or even poke throughs during / after the simulation.
Now use the morphs from the "XP Body" object to place the fabric over the hand(s). The distance should not be too large. Check the fit of the panties from different angles.
Do not hesitate to press the hand(s) - or individual fingers - into the model if you have difficulty covering the hand(s) with the morphs alone.
Before you start the simulation, you should first give the XP object a "Simulation Behavior" preset. Here I suggest the presets "XP Shrink and Stay 01" or "XP Shrink and Stay 02" (with stronger contraction).
You can apply the presets like other materials ("colors"). Now start the simulation and watch what happens.
You may need to run a few trials - i.e. experimenting - before the result is what you want. I think this is fun - I hope for you too.
.... Thumbs up
If you want to place the fingers under the slip but the thumb over the slip, you can use the "Hide" script of the Simulation Helper (Xtras) to hide the thumb from the simulation.
After the simulation you can show the thumb again and pose it slightly. You can do the same ("Hide" script) with other body parts.
.... Pull it
There are several "Pull" morphs. You can use these not only to make the slip slide down later (via simulation), but also to simulate a hand pulling the slip.
The procedure is simple: use the desired "Pull" morph, place the pulling hand (or whatever) inside the created space and start the simulation with a strongly contracting preset
like "XP Shrink and Stay 02".
Keep in mind: Sometimes the area where you want to "anchor" the fabric is too small. Then the simulation "tears" the fabric. So if it doesn't work with a single finger as a "hook" - use two or more fingers.
.... Slip it
OK - this is easy: use a "widen" or "pull" morph, start the simulation and you're done.
But wait - there is much more! E.g. you can let another part of the body (or another body) catch a slip while it is slipping down. Or you can let the slip slide so that it comes to rest on not one, but two bodies.
Experiment around - if you have the time and inclination - just for fun. You'll probably discover possibilities that even I haven't thought of yet.