IANAL. However, I have experience with this. Since this was posed as a general question, a few thoughts, the first few are not even about making the product: Get a contract agreed and signed before doing any work. The contract should name the two parties, establish a start date and termination conditions, establish the scope of the work, rate of compensation, method of delivery of source materials, indemnification of using copyrighted works, etc. Different situations use different types of contracts. If you are placing this under your creator account, you might use a licensing agreement to license the use of the adventure, otherwise you might use a work for hire contract. If you are not sure what you are doing, get a lawyer to set the contract up for you, even if it is a short-term lawyer from RocketLawyer or LegalZoom. Realize that either you or your client is probably going to wait to get paid. Nobody sells as many copies of adventures as Roll20 does with their licensed products. Most people go after the name recognition and advertising of WotC and Paizo products. Rime of the Frost Maiden is already the 13th-highest-selling adventure of all time on Roll20. <a href="https://marketplace.roll20.net/browse/search?category=Games:Adventures&sortby=popular" rel="nofollow">https://marketplace.roll20.net/browse/search?category=Games:Adventures&sortby=popular</a> Either you are paid up front for your work, and your client waits on getting enough sales to make up their investment, or you work for royalties and wait to get paid for your time. One of you is going to take the risk if the product does not sell well. If you are selling this under your account, you need to have a license for everything in the product. Ideally, your client can provide everything to you. Especially be cautious about art. If they commissioned the art for their product and have ownership over it, they can license the art to you as part of the licensing agreement. However, if they bought a bunch of stock art off of DTRPG, you will need to get your own art to use. If this is under their account, you most likely can just reuse what they put in the PDF, but they will likely need to come up with more art for all of those monsters that have a stat block but never show up in the artwork in the PDF. Now to actual "make a module" advice. This will be through a 5e lens, but much of it carries over to other products. The first part is that making a module for sale is much more consuming than setting up a homebrew game for yourself to run. For pages like you would see in a WotC adventure book, you can spend a half hour per page getting everything in Roll20 if you have all assets ready to go and are fast in your workflow, but 1 hour is more realistic. Everything in the adventure should be in the Roll20 product where possible. Text, art, maps, etc. The product needs to be something that a GM can purchase, read, and run without having to add anything of their own (aside from PC character sheets if applicable). Maps: Set up each map with as much Legacy Dynamic Lighting features as make sense. If there is an outdoor encounter during the day in an open field, don't bother with DL for that map. Since not everyone has a paid account, tokens should be set up on the GM layer unless the players should see the NPCs at the start of the encounter. NPCs: each NPC/monster in an encounter should have a fully set up character sheet linked to a full-color token. Handouts: All text from the adventure should be in handouts. The text within handouts should link to other handouts, characters, or the compendium where possible as well as send dice rolls for things like trap damage to the chat window by using the backtick (`). It's getting late, and that's what I can think of at the moment. If I think of something else, I will add it in later. Good luck.
