First take a piece of card to use as your greeting card. I think mine was just under a4 size.
Fold in half.
Keep it so that the fold is horizontal to you (the card folds up and down not side to side).

Cut out piece of card to be the zombies head. (semi circle no bigger than half of your greeting card, with teeth at bottom and a tab on either side)
Repeat for zombie jaw but slightly smaller.
Fold tabs under and push to 'lift' card forward. Stick tabs onto greeting card. **Note that you stick them so that the teeth are against the fold line of the card but not above it as they will bend when the card closes**
Enlarge pic to see what I mean.

Crease the head and chin so they stick up more and also make creases at their sides so that the card can close flat aswell.
Decorate your zombie! I did mine with felt pens.

Decorate your background and include the words 'I love your guts!' (...as you do!)

Cut out a colon-looking piece of card and colour. Cut a slit in it and slide between teeth. Stick an unseen part of it in place so long bit is hanging out of the mouth.
I decorated it more too...

Finally decorate the front of the card. I did mine really girly and inconspicously because then the zombie will be more of a surprise!
And you can make a poem of which the final line is 'I love your guts'

All done!