If you have subscriptions directly with the provider (Discovery+, Disney+, etc), then look/search under the "Streaming Channels" on your Roku device or from the online Roku Channel Store (https://channelstore.roku.com/browse) and install them to your Roku device.
Use the login credentials you normally use with your other devices to access the service after launching the appropriate channel.
If you mean how to switch your accounts so they are all paid through Roku rather than individually with the subscription provider, you will normall y have to cancel the former, and then have to sign up again through Roku.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102