Safe Harbor – what does it mean to allocate cost basis?

Situation: All of my multiple crypto transactions have been buy only on one exchange (Coinbase) and move to only one wallet Ledger. Now, during the last few years, for a few cryptos, I moved partial amounts from the same Ledger back to Coinbase and sold them. Filed taxes as appropriate. I have nothing on Coinbase; everything is in my Ledger.

I understand that I have to take screenshots of my entire portfolio on Ledger on 12/31/24 that clearly shows the dollar amount and number of coins for every crypto I own. And also, sign the Safe Harbor document specifying I plan to use Global Allocation (method: Highest Cost Allocated First).

What do I do after that? I keep reading about reconciliation and allocating cost basis to all the crypto holdings. What does that even mean? What am I supposed to do before 12/31/24? Can you please simplify for me based on this example?

All transactions between Coinbase and the same Ledger:

1. Bought 1 BTC @ $10 on 1/1/20 (Coinbase -> Ledger)
2. Bought 0.75 BTC @ $12 on 1/1/21 (Coinbase -> Ledger)
3. Bought 1 BTC @ $15 on 1/1/22 (Coinbase -> Ledger)
4. Sold 0.5 BTC @ $20 on 1/2/22 (Ledger -> Coinbase)
5. Bought 1 BTC @ $30 on 1/1/23 (Coinbase -> Ledger)