Marsan Watchcap Decrease question

[Ugg]

Good afternoon,

I am knitting a Marsan Watchcap http://www.redlipstick.net/knit/martext.html

I did not knit it on circular kneedles because I thought I have to learn to knit kn dpns some time.

I buggered up the decrease the first time I started this and so I started it again (I knitted when I should have purled and didnt notice for a while).

Does it matter where I start the decrease? The pattern says "After every marker SSK". Does that mean that I knit 18 (18 being where the marker is) and then ssk. Or do I ssk ON the 18th.
If I ssk the first stitch and then do 18 does it matter......I am a little confused and scared of buggering it up again.