Install JDK 8 on your computer and set JAVA_HOME variable pointing to the JDK installation directory (which need to be version Java 8u172 or higher but needs to be Java 8). For versions above 1.7.9, make sure to have JavaFX installed. MCreator will warn you if you don't have it. On Ubuntu, you can install it by running sudo apt install openjfx.
Download MCreator ZIP file for Mac and Linux and extract it to the preferred folder
Run MCreator by opening MCreator.jar file or by entering
java -jar MCreator.jar
in the terminal with the current location of MCreator's installation