Stalhrim is a crafting material in Skyrim: Dragonborn
Get Stalhrim by mining Stalhrim deposits.
How to forge with Stalhrim
Find NPC in Skaal Village that sends you on a quest to save the Blacksmith. Do this quest and you will be able to forge with Stalhrim
For more specific guide on how to craft an item with Stahlrim, see:
How to mine Stalhrim
You need a Nordic Pickaxe to extract Stalhrim
Where to obtain Nordic Pickaxe
Speak to Blacksmith in Raven Rock and he will send you on a quest to retrieve it
Where to find Stalhrim
Stalhrim can be mined from Stalhrim Veins
See here for locations of Stalhrim deposits:
What is Stalhrim from Dragonborn for?
Stalhrim is used for crafting