Press and hold a base letter:

Example you want to put à: The base letter is "a", press and hold the "a" button. Don't release yet. From there, an array of accents will pop up, move your finger (without releasing) to your desired character. In my example, you want an à, so move your finger to the "à", then release. You're done

If some accents do not appear that way, maybe you need to add the keyboard of that language: I don't have the ñ key at first on mine, nor it does not pop up when I press and hold the "n", only the ŋ and the ń, so I activated the Spanish Keyboard in my tablet and now I have the ñ when I press the "n"

P.S: I see you're posting this thing in French, the base letter for the ash is "a" and the base letter for the ethel is "o"

There may be a keyboard/app that could help you out! Also, I believe the Duolingo app has accent buttons, as well as the website. I'm not for sure about that though. If there is no good app, you could try doing some simple google searches.

Hope this helps! :D

Thank you Aron and MobiuS. I'll try out your suggestions.

Install a French keyboard and use the slide typing technique and the accents are added automatically. I've used it with the Google and Samsung keyboards, it's great.