Hi @claire123 may I ask, are you working with a print or solid fabric? If a print, is the design going in one direction at all?
All these little details help to make the best choice for laying out your pattern.
The ladies above have given very sound advice. Generally, you fold your fabric in half so the two grainlines (the woven edges often consisting of dots) meet. You will know if your fabric is not really on grain if the folded part bubbles, so it may (depending on your fabric and how it was cut) take a little bit of working with it to get it sitting right.
Then, depending on the direction of your print (for plain fabrics and non directional prints it doesn’t matter) you layout your pattern according to the instructions. As you become more experienced, you can play with the layout because after all it is a guide. Then pin or use pattern weights to stabilise the pieces before cutting them out.
I hope this helps and gets you sewing these cute little shorts in no time.