Use Muraitic Acid and lye to recharge. Can get messy you need chemical resistant gloves and don't breathe it in. Best to do it outside.......
It isn't too bad one you get a system going but the first few times can be a bit messy.
Now that Bulk resin available and not spending $28 a cartridge is just wasn't worth the hazzle any more.
Here is the long version article. Others have simplified the steps but need to read the long version first to fully understand:
http://www.reefs.org/library/article/g_deutschmann.html