For my lovely sister:
I basically follow the instructions on the rice package - 4 c. water to 2 c. rice. I do about 1 1/2 c. brown rice and 1/2 c. lentils, mostly because I think rice is easier to "hide" than the lentils. Add the water and let simmer about 40 minutes until the water is absorbed. This makes enough to last about two weeks, depending on how many meals have hamburger.
I know some beans and grains you need to soak, but you don't need to with lentils or rice. Sometimes I put it in the pan an let it set for several hours, and I think it does make it a little softer, but not a noticable amount.
I've thought about looking for red lentils, since they may blend in with the hamburger better. And how much hamburger you substitute is up to you. Meatloaf I did about half and half, other recipes where the meat is alone I use more meat then rice. Dishes like casseroles I can get away with less meat, especially if they have rice in to begin with. Last time I made stuffed peppers, I think I used less than 1/4# of ground beef.
However, I DON'T recommend using this in burgers, unless adding an egg would help. I ended up with half the burger on the top of the Foreman grill and half on the bottom and they just kept crumbling. We had tavern the next day. :)