Basically, a lavacast is a way of making large cobblestone structures using just lava and water to save resources and time instead of manually
building the structure yourself. Lavacasts in Minecraft are historically used to grief Minecraft Servers, but they can also be used to easily make buildings, and other cool things that need to be really big.
To make a lavacast, you going to need a solid building block like dirt (don't use anything flammable), a bucket of water, and a bucket of lava.
After you have everything you need, you're going to build a stair-like structure with your building block that is however tall you want it to be. Then, at the top of the staircase, put a block one space above the top of the staircase.
This is so that it will be easier to place our water at the end. Now place the lava under the block you just placed on top of the staircase. Wait for the lava to flow almost all the way to the end, and then place the water on top
of the block on top of the staircase. Yes, I know that was a lot of words I'm sorry.
Heres a video to help you out:
as you can probably tell, I had a lot of fun with this tutorial. go ahead and experiment and have fun with this new knowledge like I did.