Match the cooking method first: for roast/broil choose cod or orange roughy; for gentle pan-cook choose hake or pollock.
Use the listed ratio (1:1) and weigh if your recipe calls for a specific amount of halibut.
If the substitute is softer (pollock/tilapia), cook until it flakes at the thickest part, then stop—carryover will finish it.
Season the same way, but add a little extra fat or cover while cooking if your substitute tends to dry out.
Test doneness by flake test (fork gently), not just by time, because fillet thickness varies.