First, locate icons that you want to use. Generally these can be found with a quick Google search if you don't already have some locally. Once you've got them downloaded/located locally, do a "Get Info" (File > Get Info in Finder) to show the information about the icon file.
Click on the picture of the icon in the Get Info window, which will highlight the icon.
Press Cmd-C (Edit > Copy) to copy the icon onto the clipboard.
Do a Get Info on the alias that you want to change. Click on the icon in the Get Info window to highlight it. Press Cmd-V (Edit > Paste) to past the new icon into the alias.
Repeat for any icon you want to change.