No, not alcohol! It's Vaseline petroleum jelly I'm after - a handy tool to keep in the studio for this interesting technique.
All you need is a support, paint brush, paint and some Vaseline.
Daub or smear the Vaseline onto the support with your finger (one time when you won't get into trouble using that finger).
The trick is to ensure that you're careful with removing the Vaseline, and if you're planning on doing several layers, start with the very lightest colour, progressing to darker values as you go.
Remember too, that if you do not want the original support to show through at the end, you'll need to start with a wash overall before you play with the Vaseline. You cannot always guarantee your ability to eliminate it all with a careful wash...some supports defy total cleaning!