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