When offering users to sign up on your site, you actually have to give them convincing advantages that are worth it. Especially in this time of the internet where people are scared of every company stealing their data.
Here are some general ideas to implement it on any site that contains a checkout process:
As a motivator for this you can simply have a box with account benefits somewhere on your site.
(Be careful with showing them in a pop-up, as that can potentially damage your brand. They do seem to work under good conditions, though (1, 2).)
Here you can offer them the choice upon entering checkout.
1. Log in
2. Sign up
3. Continue as guest
Or you can make it easier or more "seamless" by simply adding a checkbox at the end of the data form in the checkout process (similar to the image below).
It can then say "Create an account with this data". Make it easy to spot and read though, so it does not seem like some scam.
This one is, in my opinion, the best option for the customer.
After the checkout you can show the usual confimation with "thanks....etc." and right beneath that you show an already filled out form where only the password is missing. Here you offer the customer to sign up and show him some benefits (similar to the screenshot below, but this one only shows the password fields).
At this stage, they already entered their data anyway and already paid the money, so they do not feel as threatened by it as before the checkout.