I posted the info here some time ago, but I made mine with two 9 v batteries in series.
A 5k to 10k pot is wired with the outer ends to the outer terminals of the batteries, i.e. 18v across the pot. the pot slider is your analogue ±10vdc (±9vdc), and the centre connection of your batteries is the common.
BTW, this is for analogue drives, not step/dir!