The operating system image will need to be manually flashed to a microSD card. This can be done using a third party application like Etcher or using the official Raspberry Pi Imager.
Open the imaging application and point the software to the microSD card you want to flash and where the downloaded OS file is located from the previous step.
The latest edition of Raspberry Pi Imager (as of writing, we’re at 1.6.2) has an option to install Raspberry Pi OS (Legacy). You can download the latest Raspberry Pi Imager software from the Raspberry Pi website.
Connect your microSD card, launch the Raspberry Pi imager, and set the storage device to your micro SD card. The new legacy OS can be found under Raspberry Pi OS (other) > Raspberry Pi OS (Legacy). Select write and wait for the operating system to install.
When the SD card is ready, pop it into your Pi and boot up the legacy OS. Follow the first-time setup instructions to get everything up and running as needed for your project.