I am assuming that you are familiar with squaring a bracket. If not I would encourage you to look at this short video first before tackling completing the square.
In this series of three tutorials I take you through all the different types, depending on the x2 term.

Completing the square for when you get x2 terms only

Completing the square for when you get 2x2, 3x2,… terms

Completing the square for when you get -x2, -2x2,… terms