Okay, so your three year old was playing with the ribbons in your breviary again, and now they're all pulled out and you don't remember what week in the Psalter you should be using.
Or you got out of the habit of praying the hours for a couple weeks, and want to start up again, but don't know what week it is.
So here's what you do. Look at your parish calendar. The one that hangs on the fridge or on the cellar door. See what Sunday in ordinary time it was this week. Concentrate on that number.
Is it a multiple of 4? (4,8,12,etc.) Then you will use week IV of the psalter.
Is it a multiple of 4, plus 1? (5,9,13,etc) Then you will use week I of the psalter.
Is it a multiple of 4, plus 2? (6,10,14,etc) Then you will use week II of the psalter.
4, plus 3? (Or alternately, 4 minus 1: 3,7,11,etc) Then you will use week III of the psalter.
The weeks of the holy seasons of Advent, Christmas, Lent, and Easter Always begin with week I, proceed through week IV, and then start over as needed.
Now, print this off and paste in your breviary, and you will always be able to find your place in the psalter.