Here's my breakdown of skinning levels.
-Level 1-
Prefer old version skinning
skin.ini (skin name and author)
Hit Circles (base, overlay, approach circle)
Font Numbers (default and score by basic)
Spinners (all old version parts)
Cursor (all parts)
-Level 2-
Ranking Letters (all, including small)
Countdowns (images and audio)
Skip Button
Section Judging (images and audio)
-Level 3-
Hitbursts (all)
Ranking Screen ()
Pause and Fail Screen (all parts)
Back Button
skin.ini (custom color set)
-Level 4-
Song Selection Buttons (all buttons)
Menu Button Background
Sliders (ticks, reverses, score points)
Playfield
-Level 5-
Follow Points (animated or non-animated)
Ranking Screen (all other graphics including winner )
Unranked Graphic
Slider Balls (animated or non-animated)
Warning Arrows
-Level 6-
Modifier Icons (all osu! based)
Difficulty Stars
Effect Stars
Combo Bursts (single or multiple)
Hit Circle Select (in editor)
-Level 7-
Sounds (combobursts, failing, select, back)
Score Bar (all)
skin.ini (all other options)
-Level 8-
Taiko (notes, drumrolls, sliders, or fail)
Catch the Beat (all fruit images)
-Level 9-
osu!mania (notes, hold notes, judgements)
-Level 10-
osu!mania (hit judgements)
o!m .ini's (lane sizes and playfield position)
-Level 11-
osu!mania (all other images)
Taiko Mascot (all animations)
CtB Catcher
Game Mode Icons (all sizes)
-Level 12-
Key Input Tab (all)
Scalable Buttons (all)
Tabs
-Level 13-
Use new style skinning
Spinners (all new version parts)
Particles (all)
-Level 1-
Prefer old version skinning
skin.ini (skin name and author)
Hit Circles (base, overlay, approach circle)
Font Numbers (default and score by basic)
Spinners (all old version parts)
Cursor (all parts)
-Level 2-
Ranking Letters (all, including small)
Countdowns (images and audio)
Skip Button
Section Judging (images and audio)
-Level 3-
Hitbursts (all)
Ranking Screen ()
Pause and Fail Screen (all parts)
Back Button
skin.ini (custom color set)
-Level 4-
Song Selection Buttons (all buttons)
Menu Button Background
Sliders (ticks, reverses, score points)
Playfield
-Level 5-
Follow Points (animated or non-animated)
Ranking Screen (all other graphics including winner )
Unranked Graphic
Slider Balls (animated or non-animated)
Warning Arrows
-Level 6-
Modifier Icons (all osu! based)
Difficulty Stars
Effect Stars
Combo Bursts (single or multiple)
Hit Circle Select (in editor)
-Level 7-
Sounds (combobursts, failing, select, back)
Score Bar (all)
skin.ini (all other options)
-Level 8-
Taiko (notes, drumrolls, sliders, or fail)
Catch the Beat (all fruit images)
-Level 9-
osu!mania (notes, hold notes, judgements)
-Level 10-
osu!mania (hit judgements)
o!m .ini's (lane sizes and playfield position)
-Level 11-
osu!mania (all other images)
Taiko Mascot (all animations)
CtB Catcher
Game Mode Icons (all sizes)
-Level 12-
Key Input Tab (all)
Scalable Buttons (all)
Tabs
-Level 13-
Use new style skinning
Spinners (all new version parts)
Particles (all)