Greetings!
Heroditus, I would suggest that you read through the module carefully, making note of all the various characters, as well as a matrix of the key locations. Then, separate out the characters/events of the module that are what I call "Rigid" or "Fixed." These are characters and locations that can't, or just shouldn't be changed because they would drastically change the module. Then, look at the secondary characters and locations, and see how they can fit into your own already established campaign. Look also for room to include other major characters and locations, in order to fully integrate the module into your own campaign world. For example, if there is a wizard of some kind as a enemy character in the module, then change the name, and add in an evil wizard from your own campaign. If you look at the different caves of the Fane area, see where you can add additional cave areas and locations of your own. There may even be room for a whole series of additional encounters, all with tailored characters of your own, with specially designed information in the form of books, notes, scrolls, and so on, that ties them into the main insane cult of Therezzeddune, and so on.
You can change the name of the town, or make Hommlet near a small town of your own design, from which the characters initially start from there, to travel to Hommlet, for example. In the characters in the town of Hommlet, you can add additional characters and plots, as well. Add a drug cartel, or a brothel. Perhaps you can add a large nobleman's estate on the fringe of the town, where the nobleman is actually a Necromancer, or perhaps an Alienist. You might also want to develop some romantic interests in the town for the players, as well as some other characters that can be developed into potential friends and allies.
I hope this helps!
Semper Fidelis,
SHARK
