Open your current skin folder and find "skin.ini".
Then, copy the whole thing below and paste it in a new section.
[Mania]
Keys: 7
HitPosition: 457
ColumnStart: 200
SpecialStyle: 0
UpsideDown: 0
JudgementLine: 0
ScorePosition: 370
ComboPosition: 330
LightFramePerSecond: 48
ColumnWidth: 66.66,66.66,66.66,66.66,66.66,66.66,66.66
BarlineHeight: 0
ColumnLineWidth: 0,0,0,0,0,0,0
NoteBodyStyle: 2
ColourHold: 255,255,255,255
Colour1: 0,0,0,255
Colour2: 0,0,0,255
Colour3: 0,0,0,255
Colour4: 0,0,0,255
Colour5: 0,0,0,255
Colour6: 0,0,0,255
Colour7: 0,0,0,255
KeyImage0: mania/arrows/k_left
KeyImage0D: mania/arrows/kd_left
KeyImage1: mania/arrows/k_up
KeyImage1D: mania/arrows/kd_up
KeyImage2: mania/arrows/k_right
KeyImage2D: mania/arrows/kd_right
KeyImage3: mania/arrows/k_mid
KeyImage3D: mania/arrows/kd_mid
KeyImage4: mania/arrows/k_left
KeyImage4D: mania/arrows/kd_left
KeyImage5: mania/arrows/k_down
KeyImage5D: mania/arrows/kd_down
KeyImage6: mania/arrows/k_right
KeyImage6D: mania/arrows/kd_right
NoteImage0: mania/arrows/left
NoteImage0H: mania/arrows/left
NoteImage0L: mania/arrows/base
NoteImage1: mania/arrows/up
NoteImage1H: mania/arrows/up
NoteImage1L: mania/arrows/base
NoteImage2: mania/arrows/right
NoteImage2H: mania/arrows/right
NoteImage2L: mania/arrows/base
NoteImage3: mania/arrows/mid
NoteImage3H: mania/arrows/mid
NoteImage3L: mania/arrows/base
NoteImage4: mania/arrows/left
NoteImage4H: mania/arrows/left
NoteImage4L: mania/arrows/base
NoteImage5: mania/arrows/down
NoteImage5H: mania/arrows/down
NoteImage5L: mania/arrows/base
NoteImage6: mania/arrows/right
NoteImage6H: mania/arrows/right
NoteImage6L: mania/arrows/base
NoteImage0T: mania/arrows/tail
NoteImage1T: mania/arrows/tail
NoteImage2T: mania/arrows/tail
NoteImage3T: mania/arrows/tail
NoteImage4T: mania/arrows/tail
NoteImage5T: mania/arrows/tail
NoteImage6T: mania/arrows/tail
StageRight: mania/stage/stageright
StageLeft: mania/stage/stageleft
StageBottom: mania/stage/stagebottom
StageLight: mania/stage/stagelight
StageHint: mania/stage/stagelight
Hit0: mania/hitbursts/miss
Hit50: mania/hitbursts/bad
Hit100: mania/hitbursts/good
Hit200: mania/hitbursts/great
Hit300: mania/hitbursts/excellent
Hit300g: mania/hitbursts/extraordinary
LightingN: null
LightingL: null
Don't forget to save the file and refresh the skin by pressing "Ctrl", "Alt", "Shift" and "S" keys altogether in osu!.
Edit: If you want to know how, you can directly message me or ask here right away.
It's a bit tough for beginners - no pun intended, but I'll try to explain.