With experience you will feel out your own style and design preferences, but consider this a primer. Most Dungeon Romp games will be little half-hour excursions, although you could easily expand to hours if desired.
You don’t need to jump immediately to the next plot point once each is completed. Have the players move naturally through the game, giving them freedom to explore and try things out. Often the most fun is had in unplanned ventures!
Intro
Establish the scenario. Perhaps the group has found an old ruin rumored to hold magical loot, or they’ve been hired to rescue a loved one from a monster lair. Avoid tedious meetings in taverns and “how did we meet” discussions. Just tell the players what today’s adventure is, and jump right in!
Easy Encounter
Have a fairly easy fight, weak trap, mild negotiation or other simple event early on. (If this is most players’ first game, make it a combat.) This helps get everyone familiar with the basic rules and into the mood.
Series of Encounters
Depending on how long you want to play, have about 4-6 different scenes to explore. These can be distinct areas such as rooms, wilderness locales, caverns, etc., but they don’t have to be. Just have a main focus for each encounter: such as a combat, odd weather, secret passage, magical portal, a trap, a crevice to pass, an altar to an evil god, role-played negotiation or interrogation, etc. Most combats should be Average difficulty, but at least one should be Hard. Also, try to have at least one opportunity arise in the latter half of the game for the party to purchase new equipment.
Plot Stinger
At some point the plot should shift a bit. A revelation, discovery, or other event should occur which reframes the adventure, or gives a vital bit of info, or otherwise “turns the corner” so to speak in advancing the story. Perhaps the heroes discover the identity of the main adversary but he/she/it escapes, or they find an important bit of information, or perhaps obtain an item that’s part of the quest, etc. Maybe they find themselves teleported out of the dungeon to the center of a jungle, or ambushed and captured, or there’s a major plot twist that turns everything on its head.
This event doesn’t have to be extreme or dramatic however. Just have it change the focus of the story a bit, to reinforce the feeling that progress is being made.
The Endgame
The final encounter is eventually reached. Most often, it will be a “boss monster” of Deadly difficulty, or just Hard but with some support creatures. But it doesn’t have to always be a fight; an elaborate puzzle, dramatic court case, or other creative challenge would be equally appropriate.
Try to keep this encounter going for a bit longer than usual. Have the foe try various tactics, call for help, use surprising new items or abilities, yell insults at the party, etc. Don’t be afraid to rough up the heroes substantially; it will make their victory all the sweeter. But unless they do something outright dumb or have incredibly back luck, most of the time the players should ultimately win the day.
Conclusion
Make this relatively quick and straightforward. Describe the end of the final encounter with a tad of drama, then give one or two sentences about how the world has now changed and the heroes' exploits are remembered in songs for years to come, etc.