After you and your application file have convinced the personnel manager of your suitability for the job, the second - or even third, if a second interview took place - step is the taster day.
You will presumably be nervous, which is quite normal as this is not an everyday occurrence. Try to look forward to this challenge, and be yourself. This comes across as natural rather than staged or artificial. A relaxed but still professional and interested attitude is always appreciated.
Your clothes are also important, and should suit the vacant position. Arrive punctually at the agreed place of work. Memorize the way to work or go there the day before and familiarize yourself with the environment (parking spaces, size of the company, catering facilities, etc.). On the day itself it is a big advantage to ask many questions and to look interested. Be communicative, but do not comment on everything. When doing any work, make sure that the quality is good.