Here is a short tutorial on how to create a simple sequence diagram using platuml

Final Sequence Diagram

PlantUML Source Code

@startuml
title oAuth2.0 Authorization Code Flow
<style>
sequenceDiagram {
  participant {
    FontColor white
    FontSize 14
FontStyle bold
  }
}
</style>
skinparam sequenceMessageAlign center
actor "Aniket-ResourceOwner(RO)" AS RO #005527
participant Client #0136dc
participant "Node.js-AuthorizationServer(AS)" AS AS #340281
participant "Node.js-ResourceServer(RS)" AS RS #f74a02

RO->Client: Logs in and \nClicks the Call to Action
Client-> AS:Redirects RO over browser to \nAuthorization End Point
AS->AS: AS validates client_id, redirect_uri, \nresponse_type etc
AS-->RO:AS shows Login page
RO-->AS:RO authenticates
AS-->RO:AS shows Consent page
RO-->AS:RO provides consent
AS->AS:AS generates code
AS->Client: AS redirects browser back to redirect uri \nwith code,state as url query parameter
Client<-Client: Client Verifies code\n and extracts code
Client->AS:Client sends a \nHTTPS post call to token end point
AS->AS:AS performs validations\n and marks code as used
AS->Client:AS issues access token to Client
Client->RS:Request access to resource over \n HTTPS post call by sending token
RS->RS:Validates token
RS->Client : Returns protected resource
@enduml