If you mean something like below, then you're definitely on the right track
- Type the letter and make the font large
- Turn the letter into a path (
Path menu > Object to Path)
- Ungroup the result by selecting it and then Ctrl+U (even a single letter with no 'holes' is grouped)
- Make the letter object semi-transparent (so you can see the flowed text when you do it)
- Type your words you want to put into the shape and make the font small
- Select the letter shape and the text (status bar will say 2 objects of types Path and Text)
Text menu > Flow into frame
I also left and right justified the text to make it adhere to the shape more.
The relative size of the two fonts is critical - as is the ungroup step.
You'll get big vertical gaps unless the words are short enough to fit in the width of the letter - eg the backbone of the F here.
You can download the svg file for this but it won't render in a browser I don't think, as standard svg won't support flowRoot but you can use the Convert to Text option if you need