Do you have to tap Sun Disc then get Legion to use its effect, or can you get Legion and choose whenever to tap Sun Disc to ready a specific played unit?

To use Sun Disc's effect, you must already have the Legion condition satisfied at the moment you activate (exhaust) the Sun Disc.

According to the ruling in the FAQ:

• Sun Disc's ability checks for Legion only at the moment you tap (activate) it.
• It does not look back or retroactively trigger if you gain Legion after you have already activated it.
• If you do not have Legion when you tap Sun Disc, the ability does nothing.

How to use it correctly:

1. Satisfy Legion first: You must play another Main Deck card earlier in the turn to satisfy the Legion condition.
2. Activate Sun Disc: Once Legion is active, you exhaust (tap) Sun Disc.
3. Play your unit: The next unit you play that turn will enter the battlefield ready.

Nuance regarding the turn you play Sun Disc: If Sun Disc is the first card you play in a turn, it counts itself toward the Legion requirement. This means you can play Sun Disc and immediately exhaust it to have your next unit enter ready, because the act of playing Sun Disc satisfies the Legion condition for the rest of that turn. However, if you exhaust it before playing any cards, you will not have Legion, and the ability will fail.