Step 1: Buy Subnautica (or get it for free)
Step 2: Open Subnautica
Step 3: Press play
Step 4: Create new world
Step 5: Choose survival
Step 6: Get minor head trauma
Step 7: Put out a fire
Step 8: Open PDA
Step 9: close PDA 2 seconds later
Step 10: Jump into ocean
Step 11: Get materials (Titanium, Copper, and other stuff)
Step 11.5: Get food and water when needed
Step 12: Make a new O2 tank
Step 13: go deeper to get better materials (Silver, Diamond, other stuff)
Step 14: with 2 silver make a wiring kit
Step 15: with the wiring kit and other materials make a habitat builder
Step 16: Prey you have more than 2 titanium when making a base
Step 17: Find and scan pieces for a moon-pool
Step 18: With the moon-pool blueprint make a moon-pool
Step 19: Find pieces of Mobile-Vehicle-Bay
Step 20: Make the Mobile-vehicle-bay
Step 21: Find pieces for a sea-moth
Step 22: Make a sea-moth on the mobile-vehicle-bay

Now I'm tired of typing so watch a tutorial on youtube or something.