How do I know when to decrease with this pattern?

I am doing this pattern :

Mitered Square Dishcloth

They tell you exactly when to do the first decrease, but not subsequent decreases…

Is there an easy way to tell when to start them? I have 69 stitches now and am not sure exactly when to decrease… I’m sure it’s simple enough, but it evades me.

I have done a pattern like this, but it started at the little end, and you went to a certain number of stitches or until you ran out of yarn… forget where the link to that is ATM. Anyhoo… you are at 69 stitches. Take away one stitch as your centre stitch. Divide your remainder by 2. That gives you 68/2 which is 34. Then you will have 67 stitches, minus 1, divide by 2 gives you 33. You can see the pattern emerge… you take away one from the original knit 34 for every row of decreases you do!

You do the double decrease every other row, and knit every other row.

"Continue in same pattern with first color-double decrease in center of first row, knit back."

The first row of a color gets the decrease and the next row you knit back.