The HC-05 Bluetooth module is a Plug-n-play one. Just connect it to the board and it will start working (no need to connect and disconnect pins/ports as suggested by some).

• The default PIN of a HC-05 Module is 1234
• The fast blinking LED indicates that it ready to connect
• Slow blinking LED indicates it is connected to another Bluetoooth device
• The tiny switch is for resetting/unpairing the module.

Connect it like connecting any other bluetooth device — find it from the phone, select the name (default HC-05), select pair, type in the PIN. That’s it.

Notes:

If the bluetooth module is connected to the Arduino board through RX and TX ports then the Arduino Board cannot be programmed using the USB, it will give error.
To program the Arduino while the Bluetooth is connected —  disconnect the RX and TX of the bluetooth module, load the program/code to the Arduino, connect the RX and TX of the bluetooth module

After resetting the bluetooth module or powering it off and then on — the phone will still show that the module is paired, so need to unpair first and then pair again.