If you ever need to work with loose ball bearings, you'll find that they are very hard to hold in place when you are doing whatever it is that needs to be done to keep them in place. A trick I use is to spray the area that needs to hold the ball bearings (usually the inside of a race) with shaving cream. You can then put the ball bearings into the shaving cream while you perform the assembly, and the ball bearings will be held in place.

Once you have everything assembled, rinse the assembly to remove the shaving cream and add lubricant as needed.