How should I make/sell campaign modules and one shots?

Out of curiosity how do I make a little extra cash with one of my favorite hobbies?

Well, my first suggestion is to use the Homebrewery:

homebrewery.naturalcrit.com

This uses Markdown code in the background to make authentic-looking documents. It's possibly the best and most useful thing on the internet.

You then have two obvious options (plus loads of less obvious ones which I have no experience of) - whether to publish on DMsGuild or on their partner site, DrivethruRPG.

DMsGuild allows you to use a more extensive selection of D&D IP, such as Beholders and such, which are withheld from the OGL. It lets you use a huge amount of art assets, which can make your documents look really good without extensive effort making or commissioning art. It also charges you a little more than DTRPG, but seems to generate a higher number of visitors. It also has the caveat that Wizards of the Coast (and thus, Hasbro) owns all IP published on there. This isn't to steal your stuff - it's to stop them from accidentally creating similar content, and you saying that they stole it because you published it on their website. Same reason why Netflix never even opens unsolicited scripts or stories sent to them - they don't want to be accused of stealing if they have similar creative juices. Also, on DMsGuild, you cannot make your own setting (EG Blaerun) but can make your own geography (EG the continent of Blaerun).

DrivethruRPG has a few more restrictions on use of existing IP, but then allows you to make your own settings, and largely populate them through the OGL. Alternatively, rename things and make them sufficiently different to fit. You retain ownership of the IP, and get a little bit more money from them. On the flip side, there's much less in the way of art assets (unless you buy them, because it's a free market) and less traffic on DTRPG.

Currently, I make a more consistent trickle of funds from DMs Guild, and a more sporadic one from DTRPG. Both equate to about the same amount, which allows me to pull out enough to buy a cup of tea, maybe sometimes a cake to go with it, every couple of months. That said, I have done basically no marketing with it, and feel that is my biggest downfall!

Thank you! I will look at these

This is extremely concise and helpful! Thanks for taking the time to write it up!