It is fairly straightforward.

For Android the app can be downloaded from https://github.com/pablof7z/olas/releases

For Apple iOS it is on TestFlight (you need to install the TestFlight app from the App Store first): https://testflight.apple.com/join/2FMVX2yM

Once the app is installed you can log in using your Nostr nsec (or create a new one if you prefer to keep it as a separate account). Olas is still new and has bugs but it basically works like Instagram and is pretty easy to use.