1. Any channel ID
Open the slack webpage (http://[yourteam].slack.com) and then simply open the channel. The channel ID is displayed in the browser URL
2. Any user ID
To find a user ID you need to open your browser dev-tools console and inspect the user-link in the sidebar. The user ID can be found in the HTML attribute "data-member-id"
In Slack: (1) Click on the users name and choose (2) "Show Profile". Then open the "..." menu and you see the option (3) "Copy Member-ID ..." - to get your own user ID via the UI, you have to click the Team-Logo in top-left corner and then choose "Profile & Account"
3. Team ID
Simply open the dev-tools of your browser, switch to the "Console" tab and enter the text
boot_data.team_id into the console. This will display your team ID
Still need help? Please review this quick video: