Until tomorow 16:00 you can register to vote to European parliament. You have to be registered at municipality of Prague 1, if you are resident of Prague 1 at least from 10. 4. 2019.
Address: Vodičkova 32, Praha 1 (2. floor), phone: 221 097 397
Information on the principles and the manner of voting in various languages here:
https://www.mvcr.cz/clanek/information-on-the-principles-and-manner-of-voting.aspx
You can find documents here:
https://www.mvcr.cz/clanek/volby-volby-do-evropskeho-parlamentu.aspx