Hi @aroebuck,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
The simplest way, which works perfectly with the size of your concrete slab, is to use the 3-4-5 method.
Pick the side or corner that is most important. It may run off of an existing slab or be relative to a fence and mark it with a stake or marking paint. Once this first corner is set, measure out your sides and place a temporary mark.
You can then measure diagonally across the rectangle to determine squareness. The diagonal measurements, should equal 5 metres. If it doesn't adjust your marks slightly until both diagonals equal 5 metres and then place your more permanent mark.
Here's a couple of handy guides that should help -
Let me know if you have any further questions.
Jacob
