Susannah from Headington Junior School says:
The answer to this is to start at any corner and go all the way around the outside. Then you go round the inner square from the corner you started at.
Look closely at the drawing sent in by Harriet and you'll see that some lines go over the top of others. This gives clues for which paths to follow next.
For example, start at the top of the black line and go down. See if you can follow the lines from there.