I agree with David, one of the premade one-shots would be a good start. Alternately consider Masks of Nyarlathotep and run the Peru chapter (only in the latest version of Masks) as your learning game. The advantage of the premades is that the work has been done for you. I am on the final chapter of Masks of Nyarlathotep, and its been about a year since we started, playing generally every other week for an evening - so I think we're on Session 26 with more to go. I've been r unning it using Pulp rules for the latest edition of CoC (my players like combat and Masks fits Pulp very well). I didn't run any one shots, but I had been using roll20 to run multiple campaigns over the last few years (mostly Warhammer 40k roleplaying games like Dark Heresy) and so had a good handle on roll20 as a system. Plus Masks has a prequel chapter set in Peru which gave both the players and myself some time to adapt to the new system (since most of us hadn't played CoC since the 80s/90s). Its actually quite a good adventure which sets up the campaign proper and my players really enjoyed it. Masks has been an absolute blast, but a TON of work. So much work. Each map, location, NPC has to be entered - I worked a chapter at a time basically, keeping ahead of the players. Thankfully theres a ton of material out there on the web - historic pics, player made art and maps etc etc. It was amazing the available resources on the Web for CoC and Masks in particular. I am not sure re Mountains of Madness if there is a pdf version available like there is for Masks or Orient Express. Masks has pdfs available, plus Chaosium has put out (free!) addons for Masks with handouts/maps/NPC portraits etc - all of which you can then cut and paste to create handouts, roll20 maps, tokens (I use TokenTool) etc. Roll20 has a good character sheet and compendiums - I bought the basic rules and the Grimoire of Magic - which meant the rules and spells were all there and they work well with the character sheet (although we made some allowances for Pulp). The basic rules also comes with some free oneshot modules if interested. I also found various music from the 20s to use, plus for chapters like Africa and Peru I found suitable music to help evoke the atmosphere. There is a site - Syrinscape - which has very specific sounds/music tailored to masks (and has other campaigns as well) - I thought about purchasing it but decided against it deciding to make my own (admittedly not as impressive as Syrinscape - check it out if not familiar). Finally we made a Miro 'murder' board to track the various clues. Given how long things run and the complexity of the campaign, I felt it was helpful to have this. Here is the link, although I haven't had the time/energy to update it lately. The players though basically now have reached the end of the campaign and are fairly 'zoned in' on what they need to do and who to take on. <a href="https://miro.com/app/board/o9J_lxrqs0s=/?share_link_id=774501660532" rel="nofollow">https://miro.com/app/board/o9J_lxrqs0s=/?share_link_id=774501660532</a> So, while running this campaign has been very rewarding - probably the best and most immersive campaign I've ever run - its also been a lot of work to create each map, handout, token, NPC etc etc.
