s, err = slack.New(slack.SetToken("test token retrieved from Slack"))

// Start the OAuth process

// First, generate a random state
uuid, err := random.New()
if err != nil {
  panic(err)
}
conf := &oauth2.Config{
  ClientID:     "Your client ID",
  ClientSecret: "Your client secret",
  Scopes:       []string{"client"}, // the widest scope - can be others depending on requirement
  Endpoint: oauth2.Endpoint{
    AuthURL:  "https://slack.com/oauth/authorize",
    TokenURL: "https://slack.com/api/oauth.access",
  },
}
// Store state somewhere you can use later with timestamp
// ...
url := conf.AuthCodeURL(uuid.String())
// Redirect user to the OAuth Slack page
http.Redirect(w, r, url, http.StatusFound)

// Now, handle the redirected URL after the successful authentication

state := r.FormValue("state")
code := r.FormValue("code")
errStr := r.FormValue("error")
if errStr != "" {
  WriteError(w, &Error{"oauth_err", 401, "Slack OAuth Error", errStr})
  return
}
if state == "" || code == "" {
  WriteError(w, ErrBadContentRequest)
  return
}
// Retrieve the state you saved in the first step and make sure it is not too old
// ...
token, err := slack.OAuthAccess("Your client ID", "Your client secret", code, "")
if err != nil {
  WriteError(w, &Error{"oauth_err", 401, "Slack OAuth Error", err.Error()})
  return
}
// Done - you have the token - you can save it for later use
s, err := slack.New(slack.SetToken(token.AccessToken))
if err != nil {
  panic(err)
}
// Get our own user id
test, err := s.AuthTest()
if err != nil {
  panic(err)
}