DISCLAIMER. I suppose you know what you are doing. On your own responsibility.
1. Prepare clean wrapper
Get Wineskin, make empty wrapper (atm 2.6.0) with latest engine (atm 1.7.47)
OPTIONAL make backup of the clean wrapper
2. Install winetricks
Open wrapper as a folder (right click > show package contents), run Wineskin.app
Advanced > Tools > Winetricks
Notice: during installation of winetricks read log carefully, and do what it says. It may ask to download some file and place it in cache folder then rerun winetrick.
select "dlls > msxml3", press "run"
wait until it's done.
select "dotnet45", press "run"
Now it's tricky part. If this winetrick installation fails mid-instalation, like for the reason described in notice above, your wrapper is now bad and you need to start again with the clean wrapper (after you downloaded/placed a file, if winetricks asked).
OPTIONAL
instal gdiplus winetrick. Without it some of the graphics will be missing (like buttons on settings menu), but it will add around 0.5 GB to your wrapper size.
5. Installing .NET Framework 4.5.2 update
Download this
http://download.microsoft.com/download/E/2/1/E21644B5-2DF2-47C2-91BD-63C560427900/NDP452-KB2901907-x86-x64-AllOS-ENU.exeOpen wrapper as a folder (right click > show package contents), run Wineskin.app
Advanced > Tools > Config Utility, change version to "Windows 7"
Advanced > Install Software, select downloaded file, install it.
6. Installing osu!
Download osu!
Open wrapper as a folder (right click > show package contents), run Wineskin.app
Advanced > Install Software, select downloaded file, install it.
7.Other
Customize icon, map folders with symlinks, etc.