I would invest in a pole sander which will screw onto the end of your roller extension pole. You can then smooth out the texture that most faux finishes have as it can telegraph through the new paint. Alot of faux finishes are oil based and you will need a primer that will stick to that, and there are acrylic primers that will stick but be sure it is an adhesion promoting primer and not just a regular latex primer or you could just use an oil based undercoater.

Then you are ready to apply your latex flat paint which will show up the remaining texture of the faux finish less than a paint with a sheen such as eggshell.