Typically, only one crafted modifier may be added to an item; the crafted modifier is shown in a brightercolor than regular mods. Crafting a mod follows the normal affix rules. Specifically:

The item cannot already have a mod of that type.
The item must have a prefix or suffix slot open as appropriate. This means that a second prefix or suffix cannot be crafted onto a magic item, nor a fourth prefix or suffix onto a rare item.

It is possible to craft more than one mod into an item with the of Crafting

meta-crafting mod.
Players can remove a crafted mod from an item with an Orb of Scouring. If the item already has a crafted mod, the recipe will automatically use a Orb of Scouring to add a different crafted mod.
There are also poe recipeswhich modify the number of sockets, socket colors, or links on the item. Players may use socket recipes on corrupted equipment, which will require the same amount of Vaal Orbs as the required currency.
http://www.vhpg.com/poe-currency-recipes/
Crafting recipes are discovered in various locations across multiple content.

Recipes are found in various locations in Wraeclast and Oriath, as well as maps, which Helena will appraise.
Recipes can also be found in specific rooms in the Temple of Atzoatl.
Unveiling Veiled items adds experience towards that recipe.
Socket crafting recipes can be found in the Azurite Mine at specific kinds of nodes.
Meta-crafting recipes are found at the end of The Pale Court Prophecy chains.