Each Bluetooth headset has different functionality, different sets of buttons and a different way of accomplishing the same tasks, and therefore there are no generic manuals. It may be possible to locate the manual on the manufacturer's website.
Many Bluetooth headsets enter pairing mode after the user presses a button and keeps it pressed for a specified amount of time, often from five to 10 seconds. In pairing mode, the headset can connect to a phone or computer that is searching for visible devices. Entry of a PIN may be necessary to finish the pairing process, and most headsets use either 0000 or 1111 as a code.