I have knitted few mittens from Haavisto: Sata kansanomaista kuviokudinmallia -book, and some people have asked help with thumbs. I am writing here some tips, how I am using Haavisto's patterns.

I don't know why the charts are little bit too long, and I usually start decreases earlier. If the pattern is only few rows, there is no problem, but as you see in these patterns, I've motified pattern some way:
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 #95, decreases started 16 rows earlier than in pattern and I also left 3-4 sts from left side away.
#89 knitting started in row 14

I used different kind of ribs in a cuff, now I like this knit 2 with basic color, knit 2 with charted color:
The palm I knit with some other pattern from same book, I like that they look different. Between the palm and back of the hand I knit one or three sts stripe, here is one. This stripe continues at the end, and I decrease sts around it.

The Book doesn't give any information about thumb, and I love to use same method as in the book Mostly Mittens.
In the second charted row I increase one st after/before stripe for the thumb.
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Every second row, I increase one st with charted color in both sides. When I have enough sts for thumb, I put them to waste yarn, and increase 3-5 sts. Those sts I decrease away at the next rows. Then I have same amount of sts than in the beginning.

When starting knitting the thumb, I increase again 3-5 sts to hole, and decrease 2-4 sts at the next row. When thumb is long enough, I decrease two sts /row, and rest of "knit 2 together" in a last row.

You need to check yarn, needles and stiches what you use, I already know myself, how many sts is good for choosen yarn :)

This is translated very quickly, I think there is some mistakes with words and something is still missing, but leave me a comment what is said strange, or ask If you have any more questions... I try to fix them later.