prewashing fabric before painting on it is to remove any sizing added during manufacture which may prevent the paint from adhering to the surface. It also gives it a chance to shrink, if it's going to. To test whether a piece of fabric really needs prewashing, drop a little bit of water on it. If it beads up on the surface, it needs washing. If it sinks in, so should paint.
Skip the Softener
You're trying to get rid of chemicals, not add new ones.
Go With the Flow
Wetting the fabric with clean water before painting it encourages colours to flow into each other, like in a watercolor. But don't add too much water, as it'll dilute the colors; the fabric should be damp, not soaking.
This are some tips too!