You attach to the top rings; the ones that the cheek pieces go on. the curb makes the action of the bit MUCH stronger. You can always use a flash strap which is kinder, making sure that it doesn't actually come into effect until the bit is at around 45 degrees.
Antifaz HATES curbs so I ride him without.
I don't know which gag you are talking about as there are many different types of gags. Presume you are talking about a Dutch Gag though? In which case you either use a leather curb strap or a leather strap with chain in the middle. Both of these are attached onto the top ring where the cheekpieces are also attached.