This achievement took me the longest. You need to purchase the ability to forge items through your citadel to start with. Once you've done that you need to collect three runes to forge an item. This is done by going to certain points on the map and selecting "Search for runes." from the menu.
I recommend collecting these runes from these locations since they are close together.
Rune of Jewels - Blood Moon
Rune of Gods - Valley of Kings
Rune of Fire - Free once you buy the forge.
Collect those three runes and attempt to forge an item. It works basically the same way as researching a spell. It takes time and patience since you have to clear 20 anvils to complete the puzzle. The best advice I can give is to make sure you have a possible move somewhere on the board besides where you are working. Since it's not an area you are moving around, you typically don't have to worry about if you are making the last move or not.