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

Video Thumbnail
Completing the Square (1) : ExamSolutions Maths Revision Videos

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

Video Thumbnail
Completing the Square (2) : ExamSolutions Maths Revision Videos

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

Video Thumbnail
Completing the Square (3) : ExamSolutions