The easiest method I have found is this:

I assume I will be getting rid of the original file (if possible)

I create the new files (you do not need to have a new file group, although I prefer it, its not necessary) and be sure to size appropriately.

I then Shrink the existing file, taking the option to move all data to other files in the same group.

I do not recommend this during busy times obviously. I also remind you that the initial file cannot be removed as some things will remain in there for housekeeping the server uses. But usually that is less than 10meg.

The process can be safely killed if necessary and restarted. The option in question marks the file so that SQL Server will not put anything else in it. Once the shrink is done, you should end up with balanced files and the remaining file may be removed, if it was not the initial file.