Cross-stitch embroidery patterns consist of a grid with symbols that marks the spot for crosses to stitch.
Every grid square of the pattern represents a “square” on the woven (Aida) fabric.
Each symbol represents a specific floss color and every pattern should be accompanied with a color map.
Usually you start with stitching one color at the time. Some prefer to complete all crosses of a color before they switch to the next color and some prefer to complete a smaller section completely at the time. You decide which way works best for you. Just make sure to fasten the thread when you switch color.
To keep track of where you’re going to stitch your crosses you need to count the squares in the pattern and the corresponding squares of the aida cloth/fabric.
If you somehow mess up with the counting your embroidery can end up a little crooked. Depending on how wrong it is you can either remove the thread and re-do it or you can embrace the “error” and maybe try to adapt the pattern to it.