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 (1 of 3) | ExamSolutions Maths Made Easy - youtube Video

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

Completing The Square (2 of 3) | ExamSolutions Maths Made Easy - youtube Video

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

Completing The Square (3 of 3) | ExamSolutions Maths Made Easy - youtube Video