Decide what you need most: chewy bite (barley/spelt) or softer grain bulk (brown rice/quinoa).
Measure your substitute using the 1:1 ratio, then start checking doneness 5 minutes earlier than farro would typically take.
If your recipe needs farro’s texture (salads or warm grain bowls), drain/undercook slightly and cool before mixing.
If your recipe is baking-thick or binding-heavy, use the substitute that softens well (spelt/oats) and let the batter sit briefly before baking to hydrate.
Taste-test the grain before combining with dressing or sauce; add a splash of water/broth and rewarm if it has tightened.