I would start with a google search as I am 100% confident there is going to be something useful for you to follow. I would also encourage you to seek out a team lead or dev manager for guidance as I am not sure your company would appreciate a forum architecting their SSO server/functionality for them.
I would encourage you to do some research, figure out what SSO protocal you wish to use (Ex: openid or using active directory) and then use google to figure out what your next steps for implementing. I encourage you to use google as I can asure you, you are not the first person to implement SSO and so many of your issues you are bound to encounter have surely been discussed and are a simple google search away.
asp.net core single sign on[
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GitHub - IdentityServer/IdentityServer3: OpenID Connect Provider and OAuth 2.0 Authorization Server Framework for ASP.NET 4.x/Katana[
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https://auth0.com/docs/architecture-scenarios/application/web-app-sso/implementation-aspnetcore
Now should you attempt to implement it and get stuck anywhere, if you are able to given this is for your job, feel free to come back with a clear explanation of your issue and any relevant code samples.