1. Create the Email Template: First, you'll need to create an email template that will be sent to customers. you'll use that in later and use can also edit existing email templates 

2. Create an Automated Action (Server Action): You need to create an automated action that triggers the email one day after the purchase.

3. Use a Scheduled Action for Timing : To send the email exactly one day after the purchase, you'll need to create a Scheduled Action (cron job) that will trigger the email after the specified delay.

this is a high level technical work so I strongly recommend you to use professional developer aid in this matter or pay attention to detail and read odoo automation documentation thoroughly before.