I've used Pebeo, Lumiere, Neopaque, Jacquard Textile Color, and Dye-na-flow. They're all very fun. It all depends on the look you want.
The Pebeo Setacolor can give you a wonderful watercolor effect and you can sun print with it if desired. I painted a sunset panel with this that I'm currently putting into my latest quilt. (The "pond" quilt in my profile.) I had PFD cotton on hand that I prepared myself by washing in Synthrapol so that's what I used. I spritzed it with water and used a sponge brush to apply the watered-down paint. Very fun!
The Lumiere gives a lot of shimmer. If that's what you're aiming for, go for it -- it provides lovely results as well. It sits on top of the fabric so I didn't find that you needed to prepare the fabric in any way.
I use the Neopaque and Textile Color for more detailed work and use a regular paintbrush. It stays where you put it. (I used these in my pond quilt as well.)
The Dye-na-Flow will soak into the fabric and "flow" out more. Great for making "blobs" of color.
Just make sure whatever paint you use, you follow the directions for setting it. I haven't used paints on a quilt that will be washed necessarily -- mostly wallhangings. But I still set them as directed. You never know!
If you want to prewash the muslin, I would just wash it regularly but maybe skip the fabric softener. If the teacher didn't say to prewash though, it most likely isn't a big deal.
Well, I know there are those who are more adept at this than I am but when I do any fabric painting or dyeing--I prewash the fabric--you want to get the sizing out so the fabric can take the paint and dye. Otherwise, it just sits on the fabric and when you wash it it washes away with the sizing.
I am going to a class on Painting on fabric but it just asks for muslin and didn't ask for any pre-washing or treatment. What are the usual choices we have for painting. I've heard of Pebeo setacolor. I have lumiere paints for painting but I've been afraid to use them. Thanks for any information on painting that you have. Jan