So here it is.
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Okay, first things first. The best Hex Editor I have found is called HexEdit, and you can find it here. If you have some other Hex Editor, I advise you get rid of it and use HexEdit instead, since this tutorial does not cover other Hex Editors.
The tools in HexEdit are pretty basic. When you open a map file, you should see three distinct columns, with very many characters and numbers. On the far left side is a list of addresses. Addresses are always eight characters long. These addresses are in hex (I advise you read this, so you have a solid understanding of how hex works), and relate to the position in the file where the line starts.
If you read that page, you'd know that address 00000010 is the line which starts at byte 16 (Since A, B, C, D, E, and F come before 10 in the hexidecimal numeric system). 00000020 starts at byte 32...etc.
These addresses are very useful in hex, since they are much shorter than they would be if you expressed them in decimal (This only applies to large numbers; for example, 3,184,935 in decimal is only 309,927 in hex -- but I dont want to confuse you, so I wont go into that).
By pressing command+J, you can "jump" to any part of the file, instantly. Just select the "Entry type" (decimal or hex -- since we're using hex editors, I'd keep the "hex" button selected), type in the address, click "GO," and you're off!
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The next column holds all of the Hex Code. In this column, Hex Bytes are in pairs, and coordinate with ASCII codes (Again, I advise you to read this, since it will minimize the amount of questions you might want to ask).
You will also notice something strange about this column. Unlike the Hex Address column, each row of this column is split into four equal parts, each having 4 hex bytes (8 characters in total), which are split by spaces. There are 4 dividing lines in this column, each having 4 hex bytes, since hex, as you may have guessed if you read the article I posted above, is a numeric system based on 16 (And what's four times four?).
Also, most things in the map file (Object tags and whatnot) are divisible by four, which makes it fairly easy to edit with a Hex Editor.
And finally, the third and last column, is ASCII. Since this column is filled with characters your keyboard may not be able to generate, I'd advise you to stick with editing the hex part. This column is more of a reference to what you're editing, and can be useful from time to time.
The very top of the screen has an info bar. You should see "Len: $xxxxxxxx | Type/Creator: xxxx/xxxx | Sel: $xxxxxxxx:xxxxxxxx / $xxxxxxxx."
Len is file length, in hex by default (but you can change it to decimal by going to Options, and then selecting Use Decimal Addresses).
Type/Creator is pretty self explanatory.
Sel is a bit confusing at first. The first eight numbers after "Sel:" (the one on the left side of the colon; $xxxxxxxx:xxxxxxxx) show your current position in the map file, and the second row of 8 numbers will only differ from the first numbers if you have part of the file selected. In this case, these numbers show where your selection ends. For example, if you have the first four ASCII characters selected, Sel: will look something like this: Sel: $00000000:00000004 . The number after the frontslash shows how many bytes you have selected within the file. Get it?
And again, all of these numbers will appear in decimal if you go to Options and select Use Decimal Addresses.
So there it is. My small, yet hopefully helpful tutorial on how to use HexEdit. I hope this is enough to help some people get started. I'm not going into tags or metas or anything right now, since I believe this should be enough, and there are many topics in this forum and others which do go into that.
Have fun, and I hope someone's learned something from this.
~Deox
