Here is a readme made by ChciagoJoe! It contains a very detailed installation guide and common troubleshooting

Installation
Must have Scripthook API and Lenny's Mod Loader installed for this mod to work correctly.

Scripthook API installation:

Download the latest version here: https://dev-c.com/rdr2/scripthookrdr2/
Once downloaded extract the file anywhere.
Open the extracted files directory.
You will have a 'bin' directory, a  readme.txt file, and a dev-c url link file inside.
Open the 'bin' directory.
Copy the dinput8.dll, and ScripthookRDR2.dll into the games main directory. (Optional: Should you wish to use a trainer you can also copy the NativeTrainer.asi file as well or any of the other widely available trainers for RDR2.)
NOTE: If you are asked for administrator privileges when copying the files the game directory is Read-Only.

Lenny's Mod Loader installation:

Download the latest version here: https://www.rdr2mods...mod-loader-rdr/
Once downloaded extract the file anywhere.
Open the extracted files directory.
You will have an 'Example string placement', 'Example ymap and ytyp', 'Examples by Modifiver', 'ModLoader', and 'ModManager' directories inside along with a readme.txt, and license.txt files.
Open the 'ModLoader' directory and copy all the files into the games main directory.
Should you wish to use the Lenny's Mod Manager simply copy the ModManager folder anywhere on your computer. (It does not need to be in the game directory).
NOTE: If you are asked for administrator privileges when copying the files the game directory is Read-Only.

Dynamic Seasons installation:

Extract the files anywhere.
Open the extracted files directory.
You will have a 'SeasonsVestigia', 'Seasons', and 'lml' directories along with Seasons.asi, Seasons.dat, and Seasons.ini files.
Copy all the files directory into the games main directory.



NOTE: If you are asked for administrator privileges when copying the files the game directory is Read-Only.

 

Dynamic Seasons configuration:
The Seasons.ini file is your configuration file.

Editing this for your specific use is highly recommended.

 

For using Dynamic Seasons alone:

Install ALL files as mentioned above.
Open Seasons.ini with any text editor 
Find COMPATIBILITY_MODE change the value to the right to 0 (zero) if not already zero.
For using Dynamic Seasons with VESTIGIA:

Install ALL files as mentioned above.
Open Seasons.ini with any text editor 
Find COMPATIBILITY_MODE change the value to the right to 1 (one) if not already one.
This will load the extra vestigia files.
For compatibility with other visual mods:

For other mods using visualsettings you can simply delete the corresponding file in the lml/Seasons folder.
Then open the Seasons.ini file and set the value of that function to 0 (zero).
 

Alternatively in the Lenny's Mod Manager (found in the same archive you downloaded earlier for Lenny's Mod Loader) 
you can set the mod to overwrite Dynamic Seasons by placing a check mark in the far right column of the Lenny's Mod Manager.
Lenny's Mod Loader loads mods from TOP to BOTTOM, so you will need to place the mod you wish to overwrite BELOW the other mod 
and enable 'Overwrite' with the checkmark. (NOTE: This is my understanding. I have yet to find definitive proof of Lenny's Mod Loader's priority when it comes to overwrites, but this seems to work for me)

 

Common Issues:

If seasons change, but when you load the game back up it's reverted to summer: Read-Only is probably set. Launch Rockstar Games Launcher with Admin privileges.

Changing seasons manually for one reason or another: Open the Seasons.dat file with any text editor. Adjust the first number to 1 BELOW the SEASON_LENGTH parameter 
in Seasons.ini. Relaunch the game and wait 2 minutes real world time. reboot the game for the next season to take affect. Wait in game for 1 real world minute and shut down. Repeat as needed.

The first number in my Seasons.day file doesn't change: Read-Only is probably set. Launch Rockstar Games Launcher with Admin privileges.

My graphics look really weird: Check the game with only Dynamic Seasons installed. Then Dynamic Seasons and VESTIGIA (if using VESTIGIA). 
If all is ok, then you have another mod conflicting with Dynamic Seasons and/or VESTIGIA.

The game says the season has changed, but nothing looks different. Due to the game having to load new assets and scripts, a relaunch is required for each new season. 
Simply save the game, Exit the game, and relaunch into the new season.



