First of all one has to associate with people who practice mudita in daily live and are joyful in sharing their merits. This is something one would not easy find in "outer lands" (i.e. outside a monastery).
Second, to not practice it wrong, one has to know and understand what is a good and a bad deed, a conductive attainment or a hindrance -- otherwise you could possibly think that looking at popstars, and people jumping while watching them, is a place for proper mudita. That's a place for karuna...
Mudita means to remember and honor good attainments of other beings -- where there are no beings who had not done good, or accumulate good, in the present or past. If dwelling under people with wrong view, if even such as appreciation is usual, one would just increase one's defilements.
There is a list of reasons and people who are incapable of mudita, mostly caused by stinginess (macchariya), and aside of mind connected with unskillfullness, traders are counted to be incapable for appreciation.
Some may think of reflecting on a "Buddhanature", but actually mudita is the right way to do that. Understanding "Buddhanature", seeing the reason what to appreciate in all beings so to develop mudita in the context of Buddha's good teachings is worthy to understand; and of course, people from a Bodhisatta background have a better training in the basics of mudita (yet often with wrong view, such as inherent goodness).
For more and detail explaining see: Sharing merits - Freude teilen (pattanumodana & pattidana) [Forum Guide]
And since maccharia (stinginess) or amudita makes it impossible to reach even Jhana, not to speak about path or fruits, this words might be useful in addition: What's the reason that stinginess (macchariya) would disappear?
My person missed that the question includes already the wise detail question, Sadhu!
But I don't know which aspect of living beings is worth celebrating.
What ever of the real treasures, causes for such or effects by receiving the results can be seen and traced in beings.
And what are the real treasures?
"There are these seven treasures. Which seven? The treasure of conviction, the treasure of virtue, the treasure of conscience, the treasure of concern, the treasure of listening, the treasure of generosity, the treasure of discernment. ...Dhana Sutta: Treasure
See further topic to get into detail if wished, since discussion, exchange for clarification, is not welcome here: [Q&A] How should I express Mudita or Joy for all beings?
[Since a gift with mudita is not an approval of inproper not to happiness leading attainments: this is not given for trade, exchanges or other stock increasing bond to world.]