For users
1. visit the user profile page for example this user https://soundcloud.com/relaxmeditation
2. Click the share button
3. visit the embed tab and copy the embed code
4. paste the embed code at any editor at your computer to view it and copy the numerical value after the “/users/” text. this is the user id
For Playlists
it is the same, just copy the embed code of the playlist to your editor and you should find the numerical id of the playlist after the “/playlists/” text
For playlists, you can also use the ID found in the URL.
Example (get the PLAYLIST_ID):
https://soundcloud.com/USER_NAME/sets/PLAYLIST_ID