One and two would need to be done before you flip the domain to point to DreamHost, but the only problem with waiting between one and two is that your links and images will appear broken until you finish both one and two.
Once you’ve finished both steps, your site will ‘break’ and just not work at all on the dreamhosters URL. It’s not actually broken, though, the DNS switch will fix it all
I would do them all in one sitting, just so I don’t forget anything.
Oh! And remember it can take up to 72 hours for DNS changes to take effect!