At first you'll have to gather similar sized sea shells.
Use a pencil to mark the spot for the hole.
The sea shells I had were pretty thin so it didn't take much time to make holes in them. Place the sea shell on a wooden board, do not place the shell on hard surface. Gently rotate the awl on the marked spot and apply pressure carefully.
Once the hole is made, turn the shell around. Again use the awl carefully to enlarge and give the hole a smooth round shape.
While making holes on the shell you might break some shells but don't worry. I broke 5 shells while making holes on them.
Continue making holes on the shells, it'll take around 18-22 shells for the bracelet
Done making holes on the shells? Great! Now attach jump rings with the shells through the hole. One jump ring for each shell.
Take the chain and start attaching the sea shells with the chain through the jump ring. Leave 2-3 connection rings in the start. Also make sure to leave 2-3 connection rings between 2 shells. Keep attaching the shells until it reaches the required length.
Use a ruler to measure if the bracelet has reached its required length.
Simply attach a closure, I used a lobster-claw for the closure!
You can also attach extra chains if you want to.
Use different kinds of seas shells for variation.