At right is the right size hole for planting peace poles.
With a posthole digger dig a narrow hole like the one at right.. Set the peace pole in it and then backfill it with the earth that came out of the hole, tamping as you go, perhaps tamping with a 2-by-4. Stone poles need a hole 32 inches deep. Metal peace poles need one about 20 inches deep. There is no need to reach the frost line in either case.
A narrow hole is the goal. The less the earth is disturbed around the peace pole the better it will support it.
Before digging you could mark a spot on the handle measuring from the bottom of the digger to help you judge when to stop.
Once the peace pole is in the hole, to get it plumb (which means vertical and not leaning), it can be useful to hold two levels against two different sides at the same time.
Posthole diggers can be rented from tool rental companies for about five dollars per day or purchased for around $30 at hardwares and at Amazon.com. On the web there are videos wisely telling you to wear steel toed boots when you do this.