We begin by digging a cavity out of the soil, including uprooting all weeds. This provides space for a layer of pea gravel underneath a 1-inch-thick layer of all-purpose sand or stone dust. Then, we set in the pavers for a level layout. The 4-inch x 4-inch wood collars (3 sides) and concrete patio (1 side) provide the shell for a snug fit.
We dig out the cracked and crumbled mortar between pieces of flagstone. Then we pour in mortar mix (with water) so content is soft but thick enough for a snug fit when dry.
It's water applied at a very high speed. No chemicals are used. The hose from the machine hooks to the hose from your spigot. Notice how one application turns a) soiled mortar between bricks a brilliant while and b) dirty deck wood a yellowish-beige. (this is from one of my jobs, though I no longer pressure wash for my customers)