Once your swatch is competely dry, unpin it, it should now sit flat. Take a tape measure and insert pins across the fabric to show the distance shown for your pattern’s tension (for my swatch 10cm) . Remember to use pins with large ends, normal dressmaking pins will disappear through your knitting.
Count the stiches between the pins, I have 14 which is the correct tension.
Then measure your distance again, this time vertically up the fabric, and put in your pins.
Count the rows between the pins, – again I made tension with 20 rows over 10cm (*is a smarty pants*).
I now know I have gauge and can cast on with impunity. I will also probably recheck the tension on the actual garment once I get past the first bit just to make sure.