be sure you have completed the Wing of the Dragon quests part

you get Class Script Scraps (and need to collect 50 of these)

(as per @Edenprime guide:)
- Completing normal and veteran Trials: 1 Scrap from Normal, 2 Scraps from Veteran
- Opening normal and veteran Trial weekly coffers: 1 Scrap from Normal, 2 Scraps from Veteran, 4 Scraps from Veteran HM coffers
- Appearing on Trial weekly leaderboards: 5 Scraps (per character)
- Completing normal and veteran Arenas: 1 Scrap from normal, 3 Scraps from Veteran (once per day per account)
- Appearing on an Arena weekly leaderboard: 5 Scraps (per character)
- Completing Infinite Archive Arcs: Chance of 1 Scrap from Tho’at chest
- Opening rewards for completing random daily normal and veteran Dungeons: 1 Scrap from Normal, 2 Scraps from Veteran (once per day per character)
- Opening daily Battleground reward boxes: 1 Scrap (needs confirmation if repeatable)
- Completing Master Writs: 1 Scrap (once per day per account)
- Opening reward boxes granted from completing Elder Scrolls capture quests in Cyrodiil: 4 Scraps (Needs confirmation if repeatable)
- Becoming Emperor in Cyrodiil: 25 Scraps

Cyrodiil dailies, Daily DLC World Boss quests (and you need quest, not just killing WB), and Fighters Guild dailies - these are for Signature Scripts, not for Class Mastery

For Class Mastery i every day normally did
- one Enchanter Master Writ ( 1 scrap )
- vMA on better account, nMA on lower level ones (3 or 1 scrap)
- Infinite Archive, though it is not a guaranteed drop, but i collect sets anyway