Answer by
LeheckaG (1826)
Layout/measure/mark your-fire-pit-outline.
Excavate your-fire-pit inches-below-ground-level, removing roots/flammables.
Line your-fire-pit-perimeter with non-flammable bricks/masonry/stone,
extending inches-above-ground-level;
Fill all gaps with non-flammable-materials (clay, dirt, mortar, or sand).
"Extra-points" for a metal-pot-hanger/barbeque-spit extending over-your-fire-pit.