Working with Saved Games
Working with large saved games can be problematic, as they can contain dozens of mods you've not used before and potentially thousands of assets you've not subscribed to.
This guide contains a few tips for dealing with that.
Check LSM settings
If you've set LSM to skip prefabs, you might want to turn that off while testing the save.
Using shared Steam account
This is by far the easiest way to test saves, but you need a spare gaming rig to do it.
Set up a new Steam account on the other computer
Family Share with your main account
Install C:SL on the new account (free, due to family share)
You now have a clean C:SL to test on 🎉
This way you can subscribe the save, subscribe to all its used assets and mods, enable all of them and not worry about your existing mods and assets getting in the way. Afterward, you can just "Unsubscribe All" from the second account to clear it back to vanilla ready for next session of saved game testing.
Assuming the computer can handle it, you should be able to load the save without too much pain.
For huge saves, with loads of assets/mods, you might need to increase page file depending on how much RAM the computer has.
Using your own Steam account
If you only have one computer, well, things are more difficult. :(
Create a "RESTORE" save game
Start a new game, and save it with name "RESTORE" (or similar).
This save keeps a record of all enabled mods and assets you had at the time it was created. That means you can use it to get them back later.
Note: If you have a bunch of disabled assets/mods, the Mods Listing or Assets and Mods Configuration mods might be of use.
At a later date, if you add more mods assets you can open the "RESTORE" and just save it again - it will update with all the additional mods/assets you have at that time.
Nuke your subscriptions
Unsubscribe all your subscriptions:
Go to Steam Workshop
In right sidebar, hover over Your Files drop-down and choose Subscribed Items
Click the Unsubscribe From All button
Your game is now vanilla.
Subscribe the saved game
When users share their saved game with us, they will provide a URL to that save game.
To subscribe you can do one of the following:
Web Browser: (Recommended) Just go to the URL and click Subscribe
This requires your browser to be logged in to Steam, obviously
Steam Overlay: Launch C:SL, then from main menu:
Press Shift + Tab to display Steam Overlay
Click the "Workshop" link near the top
It opens as a web page, and you can enter the URL at the top
Install auto-mod enabler
Subscribe Auto-Enable Mods mod.
Enable it.
Subscribe and enable its assets
Go in to Content Manager > Saved games
Find the saved game and click Subscribe All
Wait for like 5 years or something
Note that if there's a huge number of assets, it's unlikely they'll all get subscribed in one go (there seems to be something like 2.5k limit to how many it will do at once) - so you'll likely need several attempts to get all the assets downloaded.
Disable useless mods
The mod auto enabler should automatically enable any mods that get subscribed in the previous step.
From a testing perspective, some will not be required (disable them to reduce load time):
Aesthetic / eye candy mods
Radio stations / sound effects
You should also ask the player what mods they had enabled. The issue here is that their saved game tracks all mods that were subscribed, not the mods that were enabled. They might have a bunch of disabled mods that would conflict with their enabled mods.
Loading Screen Mod
Subscribe Loading Screen Mod
Enable it
Set its options:
Disable Load enabled assets
Enable Load used assets
Optional: Enable Save assets report
This helps minimize the number of assets that get processed when loading the save.
Page file
It is strongly recommended to have manually defined page file with following settings:
Minimum size: 1 x RAM
Maximum size: 2 x RAM
Sometimes you will need even bigger page file for the largest saves.
Endure save game testing
Load the save game and see what happens. If it fails, you'll have to wade through your log file (I suggest creating a shortcut link to the file, so you can easily open it from your desktop).
There's no easy way to deal with the issues that crop up, you just have to keep trying different things until it works.
Reverting back
Just repeat the entire process only this time with your own "RESTORE" saved game.
Attaching debugger
If you have coding skills, you can attach a debug session to the live game and set breakpoints, etc., to see exactly what's going on under the hood. For more info see: Attaching Debugger to Cities Skylines.
Credits
Thanks to everyone who contributed ideas for this guide:
krzychu124
ntoff
1Tomber
And also HUGE thanks to the supplementary mods that help us test saved games on our main accounts!