Protection is about finding a playable contract for our side or pushing the opponents into a less comfortable contract, often when the auction is about to end if you pass.
When the opponents voluntarily stop in a fit at the 1 or 2 level, it is very likely that your side also has a playable fit. There are other methods than those described here, and your partnership needs to discuss them
There are several factors to consider before protecting. Are the opponents already in an inferior contract or do you have a defence to their present contract?
1. North Opens 1C (or 1D, 1H, 1S). You are in the pass out seat.
It is usual in the pass out seat for 1NT to show a balanced 10-14 hcp, less than a direct 1NT overcall. South has a weak hand and partner is marked with some values. Protective bidding is an area where you are licenced to bid partner's values as well as your own. Whatever range you play for 1NT in the pass out seat, make sure partner knows about it!
2. You have 12-14 Balanced. Partner has bid 1NT in the pass out seat.
You have a balanced opening hand opposite partner's protective bid of 1NT (10-14). Without a 4-card major you correctly did not double North's opening bid. Your correct bid now is 2NT, an invitation for West to bid 3NT unless minimum for the 1NT bid. Do not punish partner for sticking his neck out with a marginal protective 1NT bid by bidding 3NT directly.
3. The opponents want to play in 2S (with a fit)
The opponents are about to play in 2S if you pass. If you think the contract is correct but comfortable you have to do something about it. With a 5 card suit, bid it. In this example you double to show H and a minor. 2NT would show both minor suits. Opponents now have a decision to make whether to bid on or defend. Note hcp are not important provided they are shared round the table. You are bidding partner's hcp as well as your own. You needed shape for this bid. If partner's hand had shape it was his responsibility to take action in the advance seat. See 5.
4. OR The opponents want 2 play in 2S (with a fit)
On this auction, partner's double shows hearts and clubs. Make your choice. If you have a "strong" hand, don't get excited about it. The opponents have half the hcp and your partner has stuck his neck out. NS now have to decide whether to bid on or defend.
5. The opponents MAY want 2 play in 2H (with a fit)?
At this stage West doesn't know if North is about to pass 2S or perhaps bid 4S. West has the shape to protect in the advance seat and knows that East may be about to find himself in the position of having values but no shape. There is no particular hcp requirement for this bid. West is relying on the Law of Total Tricks for protection if NS are strong and decide to double instead of bidding game. East bids spades if possible (3 cards maybe), 2NT to show both minors, or his only minor.