This is skin part for buttons that you can find in several places in osu!, like mods selection menu, beatmap options, multiplayer/match buttons etc.
As you can see, every button is made of 3 parts. Left, middle and right. You need to be careful to made them at same height. To have correctly aligned parts remember that button-left must be connected to the right side of picture workspace and button-right to the left.
While adding any pattern/objects to it, remember that in some places button is resized... it can look a little ugly.