The instructions should have come with the adapter. You'd have to give abit more info about which one you have (dual
sim adapter that is). That way anyone that has the one that you have may be able to tell you how theirs works.
But from what I have seen, you put the SIMs in the adapter, then the adapter in the
SIM slot in your phone. Then depending on which phone you are putting it in, you carefully put the battery over the adapter or the adapter over the battery.
Some of the duals I have seen, when you turn the phone on, you have a choice of which
SIM card you want to use. You will only be able to send and receive calls on the active
SIM. Any calls that come into the inactive
SIM will go to voicemail (if you have it), or just ring no answer. Either way, you don't know the other
SIM got any calls until you activate it, but turning the phone off and back on.
I have seem ads for Dual
Sim adapters that you don't have to turn the phone off to switch between the
SIM cards, but don't know anyone that has one of them yet.