Choose a site on your property for your outdoor fireplace, using the information you obtained from the local building authorities. Make sure it is the proper distance away from your home, overhead wires and underground pipes and wires.
Excavate the site with a spade to the chosen width and shape of the fireplace. Dig down a total depth of 1 foot, which will provide a sturdy foundation for the fireplace.
Check the bottom of the excavated site with a level. Remove more soil as necessary to create a level surface.
Compact the soil inside the excavated area with a manual soil tamper. Pound the broad end of the tamper repeatedly against the ground to make the soil firm and tight.
Start building the walls of the fireplace along the inner edges of the excavated site. Lay the first course of bricks into place around the inside of the hole. Set each brick end to end, with little or no gaps between the bricks.
Mix a batch of high-temperature ready-mix mortar with water, following the package instructions. A fire-resistant mortar prevents cracking and breaking when you build fires in the fireplace.
Scoop mortar from the batch with a trowel. Spread it onto the tops and sides of each brick you place in the second row. Stagger the bricks as you lay them over the first course, to create offset joints that provide sturdier walls for the fireplace.
Install additional rows of bricks in the same manner. Build the walls least 1 foot above ground level before you create an opening in the fireplace for inserting firewood.
Leave a gap at least 2 feet wide in one side of the fireplace as you build the walls up taller, until you build the opening to a height of at least 2 feet. Cap the fireplace opening with a longer fire-resistant brick. Continue laying rows of bricks, building up the fireplace walls until you are ready to start building the chimney section.
Position the chimney insert pipe over the fireplace opening. Hold the insert in place while you apply mortar on all sides with the trowel, to prevent the insert from falling out of place.
Lay courses of brick on top of the previous rows, going around the chimney insert. This helps hold the insert in place while covering it from view. Continue laying courses of brick until you cover the insert completely.
Pour gravel into the bottom of the fireplace, through the opening, until the gravel layer is 6 inches below the opening. The gravel provides drainage and a higher surface for building fires.
Allow the mortar to dry for at least one week before using the outdoor fireplace.