Your wallet shows the words only once, when it is created. That is the moment to back them up properly.
By hand, on paper or metal
- Write the words by hand, in order, numbered from 1 to 12 or 24.
- Check the spelling of each word letter by letter.
- Make sure nobody and no camera can see your screen while you write.
Test your backup
Most wallets ask you to confirm a few words. Go further: before sending real savings, check that your backup works, for example by restoring the wallet with the words and making sure the same addresses appear. A backup you have never tested is a hope, not a backup.
Paper burns, metal does not
Paper is fine to start, but it fears water and fire. For larger amounts, many people engrave or stamp their words on a steel plate made for that purpose.