How to cook orange lentils in the microwave?
I once tried and it ended up as a watery mash. What time, power, amount of water (any other things to take into account), should I use to avoid them being mushy?
It sounds like you pretty clearly had too much water, and maybe cooked too long and too hot. As for what to actually do: using a microwave doesn't mean you can't be flexible. You can figure out how to cook them the same way you could figure out how to cook many things in the microwave.
Start just enough water to cover the lentils. Cook it on high until it's nearly boiling; then reduce the power until it can keep going without boiling much. (That depends completely on your microwave; on mine it'd probably be in the 20-50% range.) Every couple minutes, have a peek. Make sure there's still enough water; stir them and see if they're done.
If you keep track of the total time, and the amount of water you had to add as you went along, you should be able to do it without much trouble next time. (And if you like, leave out the initial heating on high; that just saves some time.)
Mushiness can be caused by cooking too long, obviously, but also by cooking too hot. A rapid boil can help disintegrate things. It's especially tricky to avoid boiling in a microwave, since reducing power just makes it go off and on. Pay attention to how much it's boiling while it's on; while it's off it'll still be plenty hot.