Click on the customer you want to get the customer ID
Copy the customer ID (see screenshot below)
You can then use this value as the customerId in UiCheck code snippet.
Accessing the customerId from your database
Normally you should retrieve the customerId from your database when rendering the billing page in your subscription software. The simplest way to do that with an SQL database is to add a column name stripe_customer_id in your user table and put their the stripe customer id of each of your user.
The signature is a cryptographic mechanism that ensure your customers will only be able to see their billing information. It is a protection against forgery. You generate a different signature for every of your customers. The signature is created with the uicheck_secret. To learn how to generate the signature check the following part of the tutorial: