Hopefully the templates and the finished photo is enough to work from, basically it is a lot of careful cutting and equally careful folding. I've included three templates, one that is a full 8.5"x11", one that is more card size (folds down to 4"x5") and a full size one without the message.
I drew one of these at school with graph paper and a pencil earlier today and I had to photocopy it many, many times because kids liked it so much. I should probably have mentioned that I'm a middle school art teacher. I can't wait for them to see the version 2.0 tomorrow:)
Use a hobby knife to cut
Score all the folds before you try folding
Fold the longest folds first
after cutting and folding, glue to a sheet of card stock or other thick paper
Thanks for viewing, and please follow or favorite if you like this instructable!
Big thanks to Kate12345 for their pixellated heart design
fungus amungus for taking it further with the Zelda heart
angelkitty on deviantart for taking the time to make a vector drawing of the diamond pickaxe
and finally I found the Minecraft font here: