You can ham this up a bit, but here's the gist of it.
Two scouts enter (one on all fours if conditions allow) and move across the stage as the skit proceeds. One is the mule, and the other is the driver. A narrator stands just offstage.
Narrator: "In the heat of the Mojave Desert, the mule driver pushes his beast toward town. The first day. . ."
Mule: "Water, master, water!"
Driver: "Patience, Jackass, Patience!"
Narrator: "Still, they drive on relentlessly. The second day. . ."
Mule: "Water, master, water!"
Driver: "Patience, Jackass, Patience!"
Narrator: "Without mercy, they push to their goal.
The third day. . ."
Mule: "Water, master, water!"
Driver: "Patience, Jackass, Patience!"
Narrator: "Still far from town, they go on. The fifth day. . ."
Voice offstage: "What happened to the fourth day?"
Driver: "Patience, Jackass, Patience!"