What to do in preparing an Iphone to Sell or Give to others

Hi I've been trying not to repeat the same mistake I've done before. D: I accidentally deleted all the info in the new device because I erased all the data/restored all settings in the old device linked to the same apple ID. What steps should I do to finally free the old device from the same apple ID? It's also linked in the Find my Iphone *iCloud

Will disassociating an old device with same apple id would stop changes from being made in the new device? I am very much confused in configuring this. I hope you could help me with this.

Thank you very much.