I want to generate my own seedphrase

Most Monero wallets automatically generate a new 25 word seed for you.

I want to generate my own seed from the word list. I did this for BTC, I want to do it for XMR as well.

It seems the only way to do this is to restore a wallet from a seedphrase. I tried, but it seems impossible to find the 25th word once I have 24 words. What's the best way to generate my own seed manually ?