As mentioned, consent is just one of more legal bases, and consent is rarely the most appropriate legal basis. If you have clarified that consent is the most appropriate legal basis in your specific case, such consent must be freely given, specific, informed and unambiguous. The consent must always be obtained from the data subject before the processing begins (and therefore also before you take the picture, for example). In addition, there must be information about at least: 1) the identity of the data controller; 2) the purpose of the processing; 3) what information is being processed and 4) the right to withdraw consent. You can read more in the Danish Data Protection Agency’s detailed guidance (in Danish).
If you need a declaration of consent, please contact SDU RIO. Here you can read more about what to be aware of when taking pictures/videos.