Once you have created and saved a new Quote, then a "Share" option will appear in the buttons at the bottom of your Quote. 

Next, select the "Share" button and a popup modal will appear, presenting you with a link to "Enable QuoteAccept". Selecting this link navigates you to Settings > Accounts > Quotes in a new tab, where you can then enable the feature and Save your changes.

Alternatively, you can navigate to Settings > Accounts > Quotes and enable the feature and Save your changes. 

While in the Quote settings, you can also add a placeholder to your Quote email (see: Do Invoices and Quotes have Placeholders?), to automatically include a link to your Quote via QuoteAccept. Follow these steps to complete this update.