Please have a look at this article if you aren't familiar with the Customer Portal or don't know how to send the access link to your clients: What is the Customer Portal?
Easy communication with your clients: you can now receive customer information (personal data and arrival information) through the Web Check-in.
1. Your clients enters their Customer Portal and clicks on Start Web Check-in.
2. They then choose the person they want to check in.
3. They are required to fill in their personal details
4. ... as well as their arrival and departure information (if applicable)
5. You can add any question you would like to ask your clients to this page. If you need help with creating new fields, please have a look at this article.
6. To modify the information that will be asked from your clients in the Customer Portal, you can enable and disable the different modules in Settings > Customer Portal.