generate a publickey/private key combi.
send some small btc to the publicaddress and send it back to test it works.
send main sum btc to the publickey.
safely store the private key, without risking someone figures out what it is.
How can I best do this?
You'd be better off handing the keys to some homeless guy on the street.
Ranochigo's advice is the best. The easiest solution for the non-tech savvy is hardware wallets, but you'll have to be careful with the software updates if you do that.