Here are some steps that I would follow to "normalize" a typical D&D adventure:
1) Convert all of the other races mentioned in the adventure to human. If you have a monstrous race that is integral to the plotline and that, for whatever reason, can not be human, then review that race's abilities, appearance, etc. to remove any excess baggage so that they do not have any excess abilities not needed for the plot. But even then, consider altered humans such as mutants, etc. or go the other direction and make them even less human by making them non-humanoid (hive-minded spiders, backwards-speaking creatures of shadow, etc.) In other cases, you can eliminate a monstrous race by replacing it with humans of a different culture, skin color, language, or religion.
2) Identify any magic-user NPCs and remove as many magical levels as possible, while still maintaining the plot and the same CR. As an example, a typical adventure may insert a sorcerer or a couple of clerics as essentially guards in a castle. But the adventure would work just as well if the sorcerer were a rogue or the clerics were fighters. If they absolutely need to be spellcasters in order for the plot to work, see if you can at least replace some of their spellcaster levels with non-spellcaster levels to reduce the effect.
3) Review all magic items, including ones held by NPCs, to determine if they are absolutely needed by the plot. There are going to be balance issues with the treasure that you give your players, but that is an issue for the entire campaign, not just for the conversion of one adventure.
a - Delete any magic items not critical to the plot (such as a McGuffin that protects the bearer from the demons in the final scene.)
b - In general, replace major magic items with medium items, replace medium items with minor items, and replace minor items with alchemical items wherever an equivalent option is available.
c - Replace stat-boosting items on NPCs with direct stat boosts. Similarly, replace any skill-enhancing items with additional ranks in that skill. So a rogue with gloves of dexterity and a hat of disguise is instead a mundane rogue with a very high dex score and very high Disguise skill. This may mean that the NPC is not in line with the "core" rules, but it ends up with the same numbers as the original but all of the magical gear.
d - Replace any multi-use item with a single-use version (such as a potion) where possible.
e - Replace any general item with a more limited item (only works for one gender or race, only works at night, etc.)
f - Add negative side effects to any of the more powerful items remaining.
g - Replace any bonus-granting items with non-magical skill-enhancing "tools". For instance, boots of spider climb might be replaced with climbing gear that grants a bonus to climbing, or a remove disease potion can be medicine.
h - Provide labels for any items that can be relabeled while maintaining the same numeric functions. For instance, all +2 swords are "star-forged steel". They are non-magical, but made from a very fine material or made in a rare or magical way. It is even okay for the construction process to be "magical" but to have the resulting item be non-magical.
A lot of the above methods have the end-result that the NPCs can be as much of a challenge as they were before, but when they die, there is little or no magical loot there for the PC's to use. Finding a way to balance this so that the PC's are still as powerful as they should be is a campaign-level challenge for the DM, not specific to one adventure.
4) Remove any magical traps or effects that are not needed. Virtually every magical trap can be replaced with a mundane one with the same challenge. Magical areas usually just represent hazards that can be replaced with mundane hazards such as a blizzard or quicksand. Magical barriers can be replaced with standard walls, moats, and chasms, or similar challenges.
5) Monsters can be replaced with humans in many places, and with less fantastic monsters elsewhere. Imagine a game world with only one race (human) and only 10 selected types of "monsters" that might appear in a monster manual. Using that as your base, you could replace every other monster in the adventure with one of those 10 types, but add in templates, class levels, or other special abilities to make them unique.
6) Villainous plans or rituals in most adventures represent plot devices, such as a ritual to sacrifice hostages to raise a demon, or a plan to capture a peasant village to turn them into an undead army. These rituals or plans in many cases are an "excuse" to have the adventure. If possible, replace the plan with something more political, or with a more vague magical purpose (prophecies are always good here.) For a typical villain, often just replacing their various magical motivations (like ascending to godhood, etc.) with the classical "to get rich and powerful" motivation can point out a clear way to re-work their plan slightly to attain that different goal, and with a lot less excess magic along the way.
