The Things Stack, the network server behind The Things Network, routes the traffic from your LoRaWAN gateways. Cloud Studio IoT is the layer above it: it receives that traffic through a webhook, decodes the payload, stores it and turns it into dashboards, alarms and reports. You need two things to connect them: your instance name and an access token.
If you don't have one yet, create an application in The Things Stack and add your devices to it.
In Integrations > Webhooks > Add webhook, pick the Cloud Studio template and fill in your instance name and an access token created for this integration.
Join a device to the application and confirm that TTN is delivering its uplinks to the platform.
Create each device in Cloud Studio IoT with its address and model. Natively supported models decode themselves; for the rest, a short script does the job.