Early planning essential !
Unbalanced hand
In your first bid you told your partner quite a lot, but not enough. You've now got to complete the picture:
- more precision about your strength, because 12-19 is too vague
- do you have more than one suit ?
- do you like any suit your partner just bid ?
You must tell your partner which strength range your hand falls into, so that he can assess the next step:
- Weak. 12-15 HCP
- Strong. 16-19 HCP
No doubt, as a good player, you thought about this second bid before you made the first one. Also, for the same reason, you have a full understanding of the meaning of partner's first replies to your opening. He he !
Balanced hand
If you have 15-19 HCP, or 23+ HCP, you have still not told your partner that you are balanced. So, if he has come back with an unbalanced hand, you must now tell him you are balanced and your exact strength.
If he has come back with a limit bid saying that he is balanced, you can now simply do your sums to decide if you have a safe 25 HCP and, if so, then put it into game 3NT. Or even into Slam if he's come back with 4NT and you are on maximum HCP.
The rebids for an opening balanced hand have largely been covered when considering how to open in the first place with a balanced hand, and when considering how to reply with a balanced hand, and how to reply with an unbalanced hand.
The Bidding Cribsheet - Side 1 and Side 2 also cover these points.
Modified thinking
Sometimes your bid will be modified by what you have learned from your partner. Most especially, if your partner proposed a suit and you can see that you have a Major fit (8-cards) then you might just want to agree to that. Your partner's bid will sometimes guarantee a 4-card Major, and sometimes it will guarantee a 5-card Major.