Below is the right size hole for planting peace poles. The less earth that is disturbed the sooner the peace pole will be sturdy.
With a post-hole digger dig a narrow hole like the one above. Set the peace pole in it and then back-fill 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.
Traditionally with peace poles it is called “planting.”
Posthole diggers, like the one seen above, can be rented from tool rental companies for about five dollars per day or purchased for around $30 at places like Home Depot and Lowes and Amazon.com. On the web how-to videos wisely tell you to wear steel toed boots when you do this.
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.
What should surround the pole in the hole is the earth that came out of it. If you put sand or gravel in there instead, that would attract moisture from the surrounding soil, which would not be good. It is best to make the peace pole more or less invisible in the soil by having what is holding it up just be more of the same soil surrounding it.
Even with stone peace poles, you will be able to twist it in the hole to orient it in the direction you choose before back-filling around it.