You should start with switching off your breaker that manages the whole external lighting circuit of your house. This way, you would be able to stay secure and to be certain of no current you should test the voltage.
Now you should line up the bolt holes with the mounting bracket, so you could know which holes are to be used.
After the fixture and bolt holes are aligned, you should screw the bracket into the box, keeping in mind that the bolts should be pointed towards the outside.
The fixture should be covered with the waterproof seal to cover all wires and avoid any hazard.
Make certain that ¾ part of filling from each wire is uncovered in the wall box and repeat the same for fixture wires, if they are not in the similar condition.
Now is the time to connect the ground wire with the wiring of your fixture. Be sure that they are properly secured by a wire connector, so no loss of connection could take place.
Confirm that no wire is naked and all ends are wrapped up by electrical tape.
Now slide back all the wires in the wall box and do not forget to cover screws with the waterproof seal.
Now turn on your breaker to check your lighting and ensure that all connections are working properly.