So. We measure gauge. This involves a bit of math, nothing too horrific. I usually scratch it out on the back of an envelope or possibly a notebook, but a calculator is also a valid solution, if decimal points really scare you THAT much.
Lay out your gauge swatch. Stick a pin somewhere in the middle of it, between two stitches. Lay out a tape measure (centimeters or inches, doesn't matter; I'm running in inches just because that's where my brain is right now). Make sure the tape measure along the row, so that you are measuring across ONE row, not several. (That's my favorite mistake.) Stick the second pin in, one inch away from the first. DO NOT LOOK AT WHERE YOU PUT IT. Do not shift it a little to one side or the other to make it 'fit' or be neat.
Things to remember:
-DO NOT LOOK WHERE YOU ARE PUTTING THE PIN, YOU WILL SHIFT IT AROUND TO MAKE IT NEAT.
-MAKE SURE TO MEASURE ACROSS ONE ROW, NOT TWO OR THREE.
Write down what you get, down to two decimal places. My measurement would be 5.25 inches. Measure three or four more times. Write down each stitch count you get. Once you've got AT LEAST THREE (but five is better) measurements, average them out. (Add them all together, then divide by the number of figures you have.) That is your UNWASHED STITCH COUNT.
Now go wash your swatch, however you intend to wash your sweater. For me this means a trip through the sink or washing machine, soaking in mild detergent, then a rinse, then a spin to get out the water. I let it dry flat, wherever it's convenient.
When the swatch is dry, measure it all again for the WASHED STITCH COUNT.
The washed stitch count is the figure you are going to use for everything, in terms of the EPS math.
The unwashed stitch count is for when the sweater is halfway done, and you look at it and say "OMG IT DOESN'T LOOK RIGHT". You can measure it right there, and know whether or not it will all work out in the blocking.
For simple, casual knits like the one we're doing, I often skip the unwashed stitch count - it's most useful for tailored knits, when you're trying to fit things precisely and a half an inch is a big freaking deal. But for now, well, you guys can use the practice, so I had you do it. But you can skip it if you really want.