Your best bet is to do what I did, download a skin and look at the structure of the .ini file. It's pretty basic and straightforward.
Skin Name. This is so EVOX can find it and place it in the skin menu.
SkinName "Gran Turismo 4","Gran Turismo 4"
[Skin_Gran Turismo 4]
Skin Text, this is where you 'place' your text on top of your image. I use a 'shadow' technique here, putting down black text, and then putting some slightly offset white text directly on top of it.
Text = 60,51,1,0x000000,0,"<Time>"
Text = 59,50,1,0xFFFFFF,0,"<Time>"
Text = 60,87,1,0x000000,0,"<Temp1> <Temp2> <Temp3>"
Text = 59,86,1,0xFFFFFF,0,"<Temp1> <Temp2> <Temp3>"
Text = 580,36,1,0x000000,1,"<SpaceC>"
Text = 579,35,1,0xFFFFFF,1,"<SpaceC>"
Text = 580,66,1,0x000000,1,"<SpaceE>"
Text = 579,65,1,0xFFFFFF,1,"<SpaceE>"
Text = 580,98,1,0x000000,1,"<SpaceF>"
Text = 579,97,1,0xFFFFFF,1,"<SpaceF>"
Logo Type. You have 5 choices. 0=Large, centered, and 1-4 for smaller logos placed in either of the corners.
LogoType = 4
Images. This is where you specify the location and name of your main and load images. They must be named correctly, and in the properly named folder, or else EvoX will revert to the default skin.
Main = "\skin\Gran Turismo 4\GT4_main.jpg"
Launch = "\skin\Gran Turismo 4\GT4_load_1.jpg"
Menu bar and menu text colors.
BarColor1 = 0x28F7CD
BarColor2 = 0x28F7CD
BarAlpha = .5
MenuColor1 = 0xFFFFFF
MenuColor2 = 0xFFFFFF
You dont REALLY need to know all of this, it's better to start out using a prog to do most of the dirty work. I use Evox Skinner v 1.27 It does a pretty good job of showing exactly what the skin will look like when it's displayed on a TV (Evox cuts off a significant amount of the edge of the image(s)
If you need any more help, let me know.