Wrap your thread around the needle the same way as for a regular purl stitch. The right needle is going to enter the 2 sts from the left toward the right. There's a video for p2 tog tbl on the Glossary page that may help. Scroll down to p2tog tbl
I had to learn this one through the videos in the Glossary here, as you end up putting your needle through those 2 stitches in a funny way. The video turned out to be the only way I could figure out how to do it without adding any yarn overs.
The trick is that you have to grab those 2 stiches' back loops not 1st stitch than 2nd stitch, but 2nd stitch then 1st stitch (with the stitches numbered right to left as you look at the front of the project). Then you just purl them together.
This gives a nice twist to your decrease.
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