Before you start make the ip address of the Windows PC and OSMC device static. It is preferred to do this in your routers dhcp settings.
Make a new user profile to avoid having to store your real Windows password in plaintext.
Open a command prompt in Windows in admin mode and type
net user username password /add
Setup your Windows shares
For each folder you want to share right click, properties, sharing, advanced sharing, share this folder, permissions, give full access to the everyone user group. In the security tab, advanced, change permissions, the user will probably not be there so you would need to add it and in the adding dialog you would select a principal, advanced, search, choose the user you created, ok, and then give full control or whatever you want the access to be.
Optional
Windows settings search for “windows search settings” and in that section click on the “advanced search indexer settings”. In this settings pane you can add in the location of your media files to the search indexer. This will speed up library updates considerably.
Kodi clients
In Kodi go to videos>files>add videos>browse>add network location> and select SMB, add the ip address of your windows PC (and nothing else) in the “server name” field and the username and password from the new account you created in windows and then click OK. This will give you a new network location that is browsable so all your sources can just be added with videos>files>add videos>browse>[PC’s ip address]>
Alternatively you can add system mounts instead as mentioned above. You can also at any point add a system mount and redirect to using it with an existing library that was setup with Kodi SMB paths via path substitution.
Accessing OSMC from Windows
After installing SMB server from My OSMC from your windows box bring up a file explorer window (win+E) and in the address bar type in \\[ip address of OSMC] and hit enter. Windows should popup asking for credentials. type in “osmc” for both and click the box to save credentials. It should not ask for it again. You should at this point be looking at the home folder on the OSMC box. You can right click on the osmc folder there and click copy and then on your desktop (or wherever) right click and select paste shortcut for quick access. In the same vein if you need to ssh often you can right click on your desktop (or wherever) new>shortcut and set the path to
C:\Windows\System32\cmd.exe /k ssh osmc@[ip address of OSMC]
(there might be some slight errors above as it just kind of went through it mainly off the top of my head and typed it out)