Typically, the initiation of an FHSS communication is as follows:

The initiating party sends a request via a predefined frequency.
The receiving party sends a number, known as a seed.
The initiating party uses the number as a variable in a predefined algorithm, which calculates the sequence of frequencies that must be used. Most often the period of the frequency change is predefined, as to allow a single basestation to serve multiple connections.
The initiating party sends a synchronization signal via the first frequency in the calculated sequence, thus acknowledging to the receiving party it has correctly calculated the sequence.
The communication begins, and both the receiving and the sending party change their frequencies along the calculated order, starting at the same point in time.
In some uses, most often military, a predefined frequency-hopping sequence is negotiated, and after completing the first step the procedure is continued from number 5.