The easiest way to show this is to set a custom status, which will be shown beneath your username. To do this:

1) While on Discord click on your user icon at the bottom left of the screen.
2) Click "Set a custom status".
3) Type your preferred pronoun here.
4) If you wish, choose "Don't clear" and this will be shown permanently.

To help with this, I have uploaded a set of custom emojis showing pronouns which you can use in posts or statuses.