The most important step when switching from knitting to purling is to bring the working yarn in front of the right needle.
Insert your right needle down through the first loop on the left needle. Just like knitting, your two needles will cross cross, but this time the right needle will be in front and the left needle will be in back.
Wrap your working yarn counterclockwise around the right needle.
Push the right needle up and through the loop on the left needle.
Bring the loop on the left needle up and off the needle. Pull your working yarn to tighten, but not too tight!