I'm assuming I should use rayTraceBlocks but I'm not sure how to use it. Can anyone help?

Closest to what? From where? You need to provide sufficient context with your questions so we don't have to waste time asking for it.

I want to find how far the player is from the ground

Still not sufficient enough. Define "ground".

 

Ground can be world height in given place or ANY block that is below you (which would also count if you were high on floating island or deep down in caves.

 

No, you don't need to use rayTrace, if you just want to check what is below you you take player's pos and iterate down and check what block is on "player.posY - iteration". Now depending on "ground" definition - if ground would be ANY block that's it, but if you want to have ground height then it can get harder. Solutions may include saving world's height in given x/z into chunkData when world is generated.

The ground is the closest block downwards from the player.

The ground is the closest block downwards from the player.

Then you already have your answer: iterate from the player's position downwards until you find a solid block.

Have a look at this: https://github.com/m1k3s/parachute/blob/1.8.0/parachute/common/EntityParachute.java

Look at lines 510, 524, and 537

Look at lines 510, 524, and 537

You can link to specific lines on Github either by appending

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Thanks, I've seen people link to a specific line before, I just wasn't sure how to do it. Learn something new everyday. Hopefully some of that code was useful.