How do you purl in the front and back of a stitch ...

Hi its easy just pearl as normal but instead of taking your stitch off the needle as you would when finishing your pearl stitch,   you just turn your needle to the back as if you are going to knit a stich which is what you will be doing , what you are basically doing is knitting that stich twice . once through the front as a normal pearl stitch and then through the back as a knit stitch all you are doing is creating an extra stitch.  I hope this helps as its a lot easier to show someone-good luck

Firstly, pearl front and back is an increase not a decrease.  Take the right hand needle and put it in the stitch as if to purl, then after purling, don't remove the stitch from the needle, and purl the other side of the stitch.  You can see a video of it here:Knitting Help: Increases

The stitch you want is the Bar Increase on the Purl side. 

It would actually make an increase not a decrease. 

You would just purl the stitch like you normally would but then you will not slip the stitch off the left needle.  Instead you will be Working into the back loop of the same stitch, insert the right needle from back to front through the back loop of the stitch on the right needle.  Bring the yarn over and purl this stitch.  Don't forget to bring the yarn over....

Then drop the stitch off and you have completed the bar increase

Knit into the front of the stitch, do not slip the new stitch off the needle, insert the needle into the back of the same stitch knit it and slip both stitches off together.