You can apply for the ETA through an app launched by the UK Home Office, which is available both on the App Store and on Google Play. If you are unable to use the app you can apply for it online.
How much does in cost? Applying for the ETA costs £10 (around €12) and will remain valid for two years. The cost cannot be refunded after you apply. From April 9 2025, an ETA will cost £16 (around €18).
What do I need to apply? You will need your passport, access to your emails and a method through which you can make online payments, like a credit card or Apple Pay or Google Pay.