East opens, South (your partner) doubles for takeout, and West acts. South is promising 12+ HCP and at least 3 cards in any suit except East's.

With less than 6 HCP: bid your longest suit at the lowest level, preferring a major to a minor.

With 6+ HCP and a 5+ card suit (not East's): bid it as though South had opened that suit and you had a fit.

With 6-9 HCP, a stopper in East's suit, and no 5+ card suit: bid 1NT.

With 6-9 HCP and no 5+ card suit: as with less than 6 HCP.

With 10+ HCP (no 5+ card suit): bid your best suit at the 2 level, preferring a major to a minor.

If West has already bid: you only act with a 5+ card suit West and East haven't bid (treat it as a fit with South, as above) — otherwise Pass.