If you already have heroku account login to account settings.
Unfortunatly you need to verify you account with credicard (no charges). I just a verification.
Once the account verified, navigate to app settings which you have example.herokuapp.com
Click on add domain and enter the following items one by one
*.example.com
http://example.com
www.example.com
Save all the items. That's it!
Open your termial and type the following command
dig example.herokuapp.com
You can able to see there is an IP present at the right side. Copy that IP
ie. us-east-1-a.route.herokuapp.com. 59 IN A 54.243.91.79
Buy a desired domain from godday www.example.com
Click on manage DNS and set the copied IP on A record.
Static godday IP already maped with the A record. Edit the A record and past the copied ip.
Wait for few min.
Start browsing with your domain!!