There are two ways: On-Premise and Cloud.
On-Premise: The first thing you need to do is download the Oracle Application Express installer from the Oracle website. Once you have the installer, you will need to unzip it and then run the setup.exe file. After accepting the terms and conditions, you will need to choose the installation type. There are two options: using the Oracle Database Express Edition or using an existing Oracle database. If you choose to use an existing database, you will need to provide the hostname, port, and SID of the database. After the installation is complete, you will need to run the configuration script. This will create the necessary tables and users for Oracle Application Express.
Cloud: Open the existing autonomous database in Oracle Cloud Infrastructure. Open 'All Database Actions' where you can see the 'APEX' option. Click on 'APEX' where it will first ask to login as admin. Once you login, it will ask to create an workspace. Under that workspace, you can create users. Then logout from admin and login as developer user with the created workspace. Once you are logged-in, you can create applications.