On the left hand side under the "Help" category, click "Getting Started".

It will say this in most documents, but really the best thing you can do is test everything out yourself. Very rarely (even rarer in older systems) are two games ever alike. The processes used in hacking, taking apart, rebuilding, or generally just causing mayhem with a hex editor, or a tile editor may be the same. The way you go about it from game to game may be entirely different. Get familiar with the tools, and how things work, I know nothing about this particular game, but if you find that hacking it's too difficult, you can always switch to something else to get your feet wet.

Either way, nothing beats practice and understanding, keep at it, don't get discouraged, and you may just find yourself eventually creating a level editor for others to use; best of luck.

You'd have to figure out the level structure (likely through experimentation. There's some documents on that, recommending corruption if you aren't able to trace the ASM code), then write your own editor with a programming language (like C or BASIC, for example).

If it was easy, everyone would be doing it. However, you can use TLP to modify the CHR (graphic tiles) and you can also import bitmaps into the editor. First, you can use the tile arranger and mess around with it for awhile, trying to order the tiles. The tiles are usually scrambled for a reason and that's to optimize space. That's just to draw new tiles though. Then you have to figure out how the graphics are stored in the PRG (Program ROM) bank. There is also a palette and color attribute tables that you have to locate. Often, the level graphics are compressed to conserve space and you may have to reverse engineer the method used to compress the tiles.

Probably, you'll have to make an editor yourself, if you want it bad enough.