I cannot purl stitch!!?

I started learning how to knit the other day. I already learned cast on, and the knit stitch so my next move is the purl stitch. I tried it over and over but I just can’t get it to work. I know you have to put the needle in through the right of the loop and have the right needle over the left, then make sure the yarn is in front and make a loop, then I am supposed to be able to just slip it through the loop but every time I do it just slips off. I have a book and even looked up directions online and watched videos. Anyone know what I am doing wrong?

pull the yarn to the front first.
try the Continental style-you can check youtube for a tutorial.
and we all struggled to learn this, patience, remember it’s a craft, no stress, so go slow and then you’ll get it.
follow this link for youtube video

http://www.youtube.com/watch?v=XuRLFl36tDY

Left Handed: Scarf Pattern – k1, yo twice, & Drop Yarn Over