In text capsules, you can add custom characters by clicking on the custom character icon in the menu bar of the capsule. It's the one that looks like an omega. It includes quite a wide range of accents and other custom marks and characters.
You can also customize your keyboard for shortcuts! (I have Vista. Sorry if you have another Windows or a Mac! I hope you're still able to find it with these instructions.)
Go to Control Panel.
If you're on the newer view, click on "change keyboard" under the clocks and time section. If you're on the classic view, click on the "regional and language" option.
Make sure you're on the keyboards and languages tabs. Click on change keyboard. Scroll down to United States - International. If it's not there, click on "add" to the right and find it. It will be under "English (United States)." Expand to the "Keyboard." Then expand again. Check the "US" under the "United States - International" thingy.
Your shortcuts will be ' + e = é, ' + a = á, ~ + n = ñ, etc.
A simple way is: Hold down CTRL, and then type the character that corresponds to the accent or diacritical mark that you want to appear above (or below) the letter. Then type the letter - so Shift-CTRL and press tilde, then press n and you get - enyay MS Support has some good suggestion on this topic.