Well, if you set your keyboard to US-International, just use the right alt key and the letter you want.
There’s nothing to be ashamed of. I only asked because the answer to your question would depend upon the OS which you use.
Here's instructions on how to set up the US International Keyboard for Windows users. It's great because it allows you to continue using the keyboard as usual for English, but if you want to add a Fada to any letter, you just use the right alt key. https://support.microsoft.com/en-ca/help/306560/how-to-use-the-united-states-international-keyboard-layout-in-windows
For Users in the US who are running a more modern version of Windows, click on Start, type Language and select "Edit Language and Keyboard options". Click on English (United States), then click on Options. Click on Add a keyboard, and select United States International.
You can switch between the standard US Qwerty keyboard and US International, by either clicking on the ENG symbol next to the clock in the bottom right of your screen, or by pressing WindowsKey+Spacebar. Or you can Remove the US Qwerty keyboard once your satisfied with US International.
Duolingo users in Ireland and the UK do not need to do this, and you won't want the US International layout anyway. The Alt-Gr (the Alt key on the right of the spacebar) is already active for you, and if you press that key with your vowels, you'll get the accented version of the vowels.