To install Roku on your Firestick, you'll need to use a third-party app or service. One popular option is to use the "Downloader" app, which allows you to download and install apps from outside the Amazon Appstore. This process is relatively straightforward, but it does require a few extra steps.
The first step is to enable "Apps from Unknown Sources" on your Firestick, which will allow you to install apps from outside the Amazon Appstore. Next, you'll need to download and install the Downloader app, and then use it to download the Roku APK file. Once you've downloaded the APK file, you can install it on your Firestick and start using Roku.