After you cast on your 150 stitches, the last stitch that you cast on will be on the right needle of the circular, and the first stitch that you cast on will be on the left needle. Instead of knitting into the first stitch to close up the round, you want to knit into the loop below the first stitch. You want to continue picking up and knitting stitches along the edge of the cast on that's not already attached to the needles.
Another way of thinking about this is to look at your stitches after you've cast on. Call the loops that are on your needles the "top" of the round. The "bottom" of the round will be the lower edge, the edge that's not on your needles. Instead of knitting into the "top" of the round, you'll knit into all of the stitches in the "bottom" of the round. But since the bottom doesn't have loops on it, you have to pick up and knit.
Pick up and knit all the way along this "bottom" edge using the right needle. The "top" edge will still be attached to your left needle. That top edge is what you'll start knitting into later on (step 5 in that pattern).
I don't know whether that's any clearer than the other instructions linked already, but hope that helps!