People will work a lot harder and produce much better results if they are happy rather than resentful about what they are doing. So, make them happy about it!
Be crystal clear about what you want them to do.
Consider what benefits the other person will gain from doing that task.
Provide them with an incentive if you need to, but make sure it is something you can deliver on!
Inform the other person about the task.
Make sure to highlight how they will benefit from completing this task.