According to Article 2 of the Immigration Law the principle of freedom of movement (Law on Freedom of Movement of EU citizens/FreizügG/EU) applies to EU or EEA citizens. In order to enter Germany you simply need a passport or equivalent documentation.
As a citizen of the EU you are issued, in accordance with §5 section 1 of the Law on Freedom of Movement (FreizügG/EU), with official written confirmation of your right of abode by the relevant registration office. This written confirmation is of a purely declaratory nature. According to §4 section 1, sentence 1 of the Residence Law (AufenthG) you are exempted from the requirement for visa documentation. Consequently, citizens of the EU and EEA can work in Germany on a self-employed basis almost without restrictions. Find out more about the Schengen Agreement here.
In the case of skilled crafts or trades the Chamber of Skilled Crafts must check if the intended trade has to be registered on the handicrafts roll and if the necessary pre-conditions for this registration have been met. Further information on this can be obtained from the Cologne Chamber of Skilled Crafts at Heumarkt 12, 50667 Köln, Tel. +49 221 2022-0, https://www.hwk-koeln.de/.
Please find further information on visa requirements here: https://www.auswaertiges-amt.de/en/visa-service/visabestimmungen-node