* I'M NOT TRYING TO MAKE THE PLAYER CRAFT A CUSTOM ITEM *

 

I want to create a mod where it crafts an item for the player (if they have the block obviously). How would I force the player to make an item (by id/name - i have the block stored). So for example how would I force this to make a snow block from snowballs.

By "craft" do you mean on a crafting table? For that you  could just create a custom recipe. Since you said "force" I think you meant that just by holding something it would "craft" into something else? In that case you would just check the ItemStack in the player's hand, compare it to the things you want to be ingredients and then replace it with the output of the "recipe".

 

Thats the part I don't understand. I'm perfectly capable of checking the inventory. I'm doing it on a command so they type /craft snowblock or something and it would craft it from the snow in their inventory

The reason for this is I want it to work on servers. So i can't just remove and add different items etc.