Complete the quest "A New Order".
Make sure you have the Steel Smithing perk & talk to Sorine Jurard. She should train you in constructing a basic crossbow & steel bolts.
Locate Gunmar's Forge, you'll need it to craft most crossbow & bolt recipes in much the same way as the Skyforge is required for Nord Hero gear.
You can now start making bolts
Step 4 (Optional)
Complete an Ancient Technology quest for Sorine. Your reward will be plans for either an improved crossbow or exploding bolts. When you start getting plans for dwarven gear, get the Dwarven Smithing perk if you don't already and you plan on using them.
Repeat Step 4 until you have common & enhanced crossbow recipes and common & 3 elemental bolt recipes, all in both steel & dwarven varieties.