To build a shrine, you will need the following items:
- x9 Netherrack
- x4 Stone brick wall
- x4 Skeleton skull
- x1 Fire source (flint and steel, fire charge, etc.)
After obtaining those, build a 3x3 floor using the netherrack and put a stone brick wall on top of each corner of it. Now put a skeleton skull on each of the walls and light the block in the center on fire.
When you are done building the shrine, you can throw items into the fire to sacrifice them. See Item Info in the navigation on top for more.
Please note that there is a 30-minute per-player cooldown for using the shrines. Shrines themselves do NOT have any cooldowns. Players are notified when their shrine cooldown is over. Alternatively, you can always sacrifice a clock, it will tell you the remaining cooldown while also not being consumed.