Measure your flags
Common sizes for patio slabs are 600 mm and 450 mm.
Make sure the start of your tape measure is flush with the edge of your flags (don't include your grout lines)
Make sure your tape measure is flat and straight
If the edges of your flags are uneven, work with the smallest measurement.
There are three different options for you to choose from.
WORK WITH YOUR FLAGS
WORK AGAINST YOUR GROUT LINES
To create the illusion of straight laid flags, you can work against your grout lines. You can see how the grout line cuts across the middle of the design. Once the floor is complete this isn't obvious.
Alternate between stenciling within your flags (Row 1) and then stenciling across the grout lines (Row 2). This creates the illusion of straight laid flags.