Redirect all urls directly with the new path to the new domain. This is the best option, as you then only need one 301 redirect.
First redirect everything with the old path to the new domain and then redirect to the new path with Shopify’s redirect function and another redirect. That is then two redirects. If SEO backlinks play a role, this is not so good, otherwise it wouldn’t matter.
If you are on Apache, then the first possibility will need a big .htaccess on www.eatxactlycakes.com. You must create a mapping old url → new url and list everything in .htaccess. That could look as follows then.
You have to create a csv file in which the old paths and the new paths are specified. And this file must then be imported into Shopify.
Your question about how long to keep the old domain: If you don’t have a significant number of SEO-backlinks, then you can give up the old domain after Google has made the switch. I.e.: After some weeks.
If you have important backlinks, then it will be necessary to keep the old domain as long as you want to keep the backlinks.
You can learn more on how to connect your existing domain to point to your new Shopify website. That way, whenever someone clicks on www.eatxactlycakes.com it will take them to their brand new Shopify store. You can keep this domain to redirect until it’s expired.
I’m curious to learn more about the store and what type of products or services they’ll be offering. Just so you know, we are not only here to help out with tricky technical questions but also to help you grow your business and provide marketing tips, so if you have any questions, just shout!