Try this.
Knit a few rows of garter stitch on needles you feel comfortable with, say 8s(4mm) and double knitting.
Now prepare to purl.
Slip the first loop on the lefthand needle onto the right hand needle inserting the needle from the BACK. Now look closely at the loops on the lefthand needle; the front bit is to the right of the back, ie it slopes from back to front from left to right. From the BACK, impale the first loop, towards you. The wool coming from your slipped first stitch is in front of your work, so put it between the needles from right to left, and slip the right needle under the left, taking the yarn with it, and drop the loop on the left needle off.The new stitch will look like a little bump in front of the needle, facing you. Repeat to the end of the row.
If you have to take your hand off the needle, never mind. The important thing is to work out how to form the stitch.
Knit the next row, and try again!
Don’t worry about the rights and wrongs of sliping the first stitch, when you’ve mastered purling you can decide what you think about this.
I remember having huge difficulty learning to purl, but I was only about eleven, and oh God, that’s 40 years ago.
Motse Stanley’s book is brilliant, it’s often on Amazon. Can’t remember the title, something like “Knitting!”