To draw a line over letters isn't as simple as it should be. It's easy to underline letters using styled text, but there's no simple shortcut for drawing lines over letters, unless you want it over a full line, in which case you use Rule Above. Here are two workarounds:
The first way: Type your letter, followed by an underscore. Place the cursor between the letter and the underscore, and reduce the tracking to perhaps -50 to -100 so that it's directly under the letter. Hold down shift and press the right arrow to select the underscore. Now increase the baseline shift until it's over the letter. A little awkward, but it works.
Second way: Physically draw a line over the letters. Easy enough, but if your text shifts in any way, you'll have to move all your lines.