Texture packs

If I wanted to override a texture for a block, how would I do it. I want to override one block while using the textures of the client’s texture pack for the rest of the blocks.

 

Just send resource pack to the player that just has that one block in it.

 

I mean for the formatting of creating the texture pack. I’m pretty new with creating texture packs.

 

You can take a look at the default texture pack here: https://www.minecraft.net/addons/ (Direct Link)
Note: You can and should delete all textures you don't change, Minecraft will automatically use default ones and it would be a copyright violation and bandwidth waste to include them.
Alternatively you can check out this tutorial in the Minecraft Wiki: https://minecraft.gamepedia.com/Tutorials/Creating_resource_pack_add-ons

 

There is a plugin on poggit called Pen which does the same thing you are describing. I recommend you look at it's source code to figure out what you need to do