How do I sign in to imessage with someone elses apple ID

Settings>Messages>Send & Receive> Sign in with Apple ID. If another Apple ID is showing you will need to tap on the account, sign out and sign in with the Apple ID you want.

see maybe i am not getting something here but why do you need to sign into iMessage with SOMEONE ELSE's Apple ID? - It is doable, but you would need their apple id and password; doing this will seriously hose up your own iMessage capability, Please explain what you are trying to achieve here?

im sorry i dont understand you would like to use someones a,id for imessage

you would need to go in to facetime and sign it to the apple id