Okay, this will be a literal "Frequently asked questions," not a full tutorial. Ask a question and I can answer it here.
How do I make a skin?
Oh boy. If you have to ask, it's going to be tricky. First off, download the "template skin for skin makers" through Osume, and read the readme.txt and all the notes in the skin.ini file. Then download one, several, or even all of the other available skins, and look at what they do, comparing what you see in the game to the files in the folder. Then make a copy of the template skin folder, edit the skin.ini to fit your skin, and start playing around with the pngs in Photoshop or a similar program (You can't make a decent skin in MS Paint, sorry. You need something that can create transparent png files). Experimentation is the best way to learn your way around, and once you think you have something worth sharing (or if you really need help) zip up the folder (right click the folder and hit send to -> compressed/zipped folder), delete any files that you haven't edited (to save space) and post it in a new thread here as an attachment, with some screenshots. The skinning community (mostly me, sigh) will be happy to lend a hand.
The background changes color! Why?
Because Peppy says so. Beatmappers can select a background color when mapping a song, so your background will be overlayed with that color. You should probably just make your playfield.png black and white, but you can add a hint of color. Just be aware that it will be recolored for gameplay. The menu-button-background works the same way.
The same rules apply to the approachcircle.png and hitcircle.png, which will change color along with the combo.
The countdown is hard to skin properly.
That's not a question, but it's true. Count1.png will be the bottom layer, so you have some leeway with that, but for 2 and 3, you should follow either the template skin or the EBO skin as closely as possible to avoid a mess. These sprites are aligned based on their left border, so variations in shape and size will mess everything up.
Why isn't anybody commenting on my skin?
There are a few possibilities:
A) It sucks
B) The topic is too obscure, so people don't bother with it
C) For the most part, only skinners browse the skinning section, so don't assume it's A or B. Sometimes nobody comments because nobody has checked it out. Be patient, ask about it in IRC, and keep improving your skin even if nobody is saying anything.
Also, make sure you post some screenshots, keep your first post updated with the most recent download, and bump your thread when you update, so people know what to expect before they download.
What do I have to do to get my skin added to the updater?
This isn't a very democratic process, as it's largely up to me and Peppy's whims. But to increase your odds of being "published" in the updater, make sure your skin is:
A) Complete. You don't need to skin every little thing, but the gameplay sprites, at the very least, should all match the theme.
2) Fun. It should be fun to play Osu using your skin. Anything too distracting, obnoxious, or ugly might ruin the experience, even if it does fit the theme, so make sure the priority is on making it enjoyable to play with.
3) Clean. I'm a stickler for this. Make sure your sprites have smooth edges, so they don't look like crap when Osu resizes them. I suggest using an outer glow or drop shadow when appropriate. Also, delete any remnants of the template skin. They waste space and make your skin look unfinished when I browse the folder.
I'll try to organize this better once I have some more questions. So ask away.
How do I make a skin?
Oh boy. If you have to ask, it's going to be tricky. First off, download the "template skin for skin makers" through Osume, and read the readme.txt and all the notes in the skin.ini file. Then download one, several, or even all of the other available skins, and look at what they do, comparing what you see in the game to the files in the folder. Then make a copy of the template skin folder, edit the skin.ini to fit your skin, and start playing around with the pngs in Photoshop or a similar program (You can't make a decent skin in MS Paint, sorry. You need something that can create transparent png files). Experimentation is the best way to learn your way around, and once you think you have something worth sharing (or if you really need help) zip up the folder (right click the folder and hit send to -> compressed/zipped folder), delete any files that you haven't edited (to save space) and post it in a new thread here as an attachment, with some screenshots. The skinning community (mostly me, sigh) will be happy to lend a hand.
The background changes color! Why?
Because Peppy says so. Beatmappers can select a background color when mapping a song, so your background will be overlayed with that color. You should probably just make your playfield.png black and white, but you can add a hint of color. Just be aware that it will be recolored for gameplay. The menu-button-background works the same way.
The same rules apply to the approachcircle.png and hitcircle.png, which will change color along with the combo.
The countdown is hard to skin properly.
That's not a question, but it's true. Count1.png will be the bottom layer, so you have some leeway with that, but for 2 and 3, you should follow either the template skin or the EBO skin as closely as possible to avoid a mess. These sprites are aligned based on their left border, so variations in shape and size will mess everything up.
Why isn't anybody commenting on my skin?
There are a few possibilities:
A) It sucks
B) The topic is too obscure, so people don't bother with it
C) For the most part, only skinners browse the skinning section, so don't assume it's A or B. Sometimes nobody comments because nobody has checked it out. Be patient, ask about it in IRC, and keep improving your skin even if nobody is saying anything.
Also, make sure you post some screenshots, keep your first post updated with the most recent download, and bump your thread when you update, so people know what to expect before they download.
What do I have to do to get my skin added to the updater?
This isn't a very democratic process, as it's largely up to me and Peppy's whims. But to increase your odds of being "published" in the updater, make sure your skin is:
A) Complete. You don't need to skin every little thing, but the gameplay sprites, at the very least, should all match the theme.
2) Fun. It should be fun to play Osu using your skin. Anything too distracting, obnoxious, or ugly might ruin the experience, even if it does fit the theme, so make sure the priority is on making it enjoyable to play with.
3) Clean. I'm a stickler for this. Make sure your sprites have smooth edges, so they don't look like crap when Osu resizes them. I suggest using an outer glow or drop shadow when appropriate. Also, delete any remnants of the template skin. They waste space and make your skin look unfinished when I browse the folder.
I'll try to organize this better once I have some more questions. So ask away.
) , the fruits-pear is blank and that the fruits-pear-overlay isn't... and then when I play catch the fruits, the fruits change colours? @@" *sadface* 
how do i add a skin? i try to download it but its just a rar file. (i'm trying to download the kirby osu skin, BTW.)