The most basic way of getting into developing your own skins is to open the existing ones and edit the bitmaps using Paint or some other graphics editing program.
When you are running Samplitude, to see where the control tags are for the mixer, press Ctrl+Shift+Alt+M to see the boundarys. This can help in identifying what to change in the .ini file, if you are not just redrawing things in their existing location.
Clicking on any mixer component will bring up it's bitmap location on the Title Bar. Move the control position with CTRL+Cursor-Buttons. Edit the control size with SHIFT+Cursor-Buttons. Save Mixer state with CTRL+POS1.
Attention: This direct mixer skin editing was removed by MAGIX in all versions from 8.21. Use previous versions for direct editing.