Open up your image editing software and create a new blank document, set the width and height to your chosen size listed in "Basic Minecraft Textures" and the resolution to 72ppi.
This document should be in RGB colour mode by default.
Now that you've created your new document this next part is up to you! you must get creative and using the tools at your disposal draw out your image.
lets say we're working on my Egyptian textures, I'd start by using my existing marble textures, then I would draw hieroglyphs on a new layer with the pencil tool.
try to work in layers if you can, you want to make sure you can always go back if you dont like what you've made, Photoshop by default only has 20 undos then you're stuck with what you've made 20 undos ago... you can up this setting in it's preferences if you wish
once you've got your basic design, go ahead an add detail to it, you dont want a boring texture right?
I use the burn tool, set it at a small diameter, then burn gently on the gold part.
Then I use the dodge tool to make highlights in the same way, but both of these are very light tools, so I made a new layer and used my paint brush in the same way but with white and black paint to make the highlights and shadows, makes it look as if it's partially 3D and the gold is shiny
Now that you've finished your shiny new texture! save it as a PNG with the block's name as per the Minecraft naming system, this Egyptian one will be "black_concrete.png" (and remember no capital letters).
Now you need to place it in the correct location, navigate back to your folders you created earlier, place this texture in:
/assets/minecraft/textures/block
Now when you load the game up and select your resource pack it will have that texture on that block!
PRO TIPS!
Because your resource pack is still a folder and not a .zip you can work on that texture and overwrite it in that folder, then in game to update it to the newest version hold F3 and press T, the game will show a loading screen and load up all the textures again, this doesn't work if you are using a zipped resource pack.
Something else to know, some blocks feature a specific top/bottom texture. it would be wise to get yourself a copy of the original default Minecraft Resource pack, and have a look at the textures given in it, like for example quarts_pillar has a side and a top texture, meaning you'll have to create two textures for that block.
Having a copy of the default resource pack is handy, you can go through and find all the textures that are available for you to change within the game, and even use them as starting points for your textures.
If you find that it is not loading in game, there is a number of things for you to check
Is the resource pack selected in the options menu? it must be on the right side and above the default resource pack.
Are your folders named correctly?, there must be no capitals, and remember the last folder is "block" not "blocks"
Is your texture file named correctly?, it must be per the Minecraft naming system, you can google these or in game hold F3 and press H then hover over blocks in the creative menu, ignore the "minecraft:". Your file might be named "end stone bricks" but it actually needs "end_stone_bricks" with the underscores
Is it a .png?
Is it the correct size?, it must be a multiple of 16x16, (e.g. it cannot be 17x17)
These things may seem simple, but they trip up the best of us as we are not expecting something so simple to be the problem.
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