What mistakes did you make, and what advice do you have for others to be careful?
If it's about bitcoin in particular, I would say that ive never made a mistake with my wallet seed since I got into crypto in 2016, all the bitcoin wallets I've used since then are still intact till today, non have been hacked, and non have I forgotten it's seed, I have a booklet were I wrote all the seed phrases of all the bitcoin wallets I've used, and still using, from the past to present, and if at any time, I decide to go back any wallet, I just up the seed and restore that wallet.
What are the best and worst ways of storing a seed in your opinion?
Well, based on my experience so far, I would say that the best way is to write them down On a piece of paper, and a book or jotter you and you alone have access to, and then keep it some where safe, though, if you ask me, I did say that this method is not a perfect one, but it does not, it's been working for me since 2016.
And, I think the waste way to store your seed is to store them online.