Pretty easy depending on size mine is 3 ft from center all the way around (mine is 6 ft across from 1 side to other).I measured out 3 ft from the center all around then sprayed with marking spray paint.You may want a smaller one,say two ft from center which would be a 4ft fire pit.
I dug it out all the way around and about 2-3 ft deep for the ashes that build up.Took a rake after the hole was dug and smoothed out the sides,went to a store and bought 29 or so retaining wall bricks keystone,put them in a circle and spaced them out about 2-3 inches and I went two layers high.
This is very simple and took me more time at the store picking out the keystone than it did doing the whole project.