Registration for a lab class is only possible in the specified registration phase via the myFH portal. Make it clear beforehand which lab classes you would like to take part in. The curriculum for your degree program will give you a good direction for this.
The type of registration differs roughly between lab classes in the Bachelor and in the Master.
Note
You can find a lot of information about the lab classes in the myFH portal. In addition, further details (e.g. division into teams) are often published on the pages of the respective laboratory or the ILIAS course on the lab class.