I find it easier to sand while the stem is straight, so I go up to 220grit (picture 1) before bending the stem. In picture 2 I have inserted a pipe cleaner and I am beginning to heat the stem with a heat gun. The stem should be attached to the stummel so you can judge what curve looks best. Take your time with this and keep the stem moving at all times to avoid melting it, especially when using acrylic. When the stem gets softened up enough simply use the pipe cleaner to pull the stem down and get the curve you want and hold it there until the stem cools.

       

Here I have sanded the stem to 600 grit and am ready to buff it. Acrylic stems need only be sanded to 400 grit and then buffed.

I start buffing with brown compound. Picture 2 is after the initial buffing and I will next move to white compound. I buff at 2100rpm and just like in heating the stem for bending you should keep the stem moving to avoid heat build up. Also, don't press to hard on the stem or you can partially melt it, more of a problem with acrylic, but you can easily ruin the buttons on even a vulcanite stem. If you find yourself having to press hard to get out some of the marks in the material you probably haven't sanded well enough. After buffing I use a clean wheel to remove any excess compound left on the stem. With acrylic you could stop here but this stem is vulcanite so I next move to the carnauba wheel and apply a coat of wax over the entire stem. 

   

Finally, buff with a clean wheel and the stem is finished.