You are not opening it with the right program. You can't just double-click, you have to right click and select "open with," then choose the .exe program you wish to use (this is outlined a little more under FAQâ†’C) I recommend downloading Winrar or 7-zip to do this.
---MSPaint does not support opacity (see-through stuff) so it just fills in all the transparent with white. Technically, you could use it, but doors, torches, rails, ladders, and everything else that doesn't take up the full 16x16 space will have white in the empty parts.
I prefer photoshop, but you could use gimp or paint.net (both free). Just google these names.
B) I don't have a "minecraft.jar", only a "minecraft"!
---You most likely don't have file extensions shown. The "minecraft" file will work exactly the same, but I find it best to enable file extensions on my computer. You don't have to do this, but if you want to, I'll explain it. On Windows, go to the control panel. Search "folder options" and click. Under "view" uncheck the option that says "hide extensions for known file types," and click "apply."
C) When I try to open the "minecraft.jar," it says "invalid or corrupt jarfile"
---This means you aren't opening it up with the right program. Your computer is trying to open it up as a program (which won't work), but you want to open it as a folder. The fix is written earlier in the description labelled "IF YOU CANNOT OPEN MINECRAF.JAR."
D) Winrar (or 7-zip) isn't in the list when I click "open with."
---You have to select "choose default program" at the bottom of the list. From there, click "browse." It should take you to your program files. Choose the Winrar (or 7-zip) folder, then find the .exe for that program. You should search for "WinRAR.exe" or "7-zip.exe" and click "open" once it's selected. Now it's in the list, and the default for all .jar files. If you don't want it to be the default, uncheck the "Always use the selected program to open this kind of file" box.
Thanks for watching and all the positive feedback! If you have a suggestion for another MC tutorial, message me and I'll see if I can make it.