Crochet Into Front Or Back Post

Stitches are worked around the post in the previous row/round. This tutorial shows double crochet (US) but the principle is the same for other stitches.
Crochet Into Front or Back Post
Front Post:
Steps 1-2: Yarn over, insert hook from front to back around post of next stitch, yarn over.
Step 3: Draw up a loop, yarn over and draw through the two loops on hook. 
Step 4: Yarn over and draw through two loops on hook.
Step 5. Ta da!
Back Post: 
Steps 1-2: Yarn over, insert hook from front to back around the post of next stitch, yarn over. 
Step 3: Draw up a loop, yarn over and draw through the two loops on hook.
Step 4: Yarn over and draw through the two loops on hook.
Step 5: Ta da!