To send party invitations using SendGrid and Java, one needs to set up a Twilio account, register an email sender, add SendGrid dependencies, customize dynamic templates with placeholders for personalized content, configure environment variables, integrate SendGrid's API with Java code, and implement best practices such as using a recognizable sender name, personalizing the email content, and including an unsubscribe link. The Java code provided uses dynamic templates to personalize invitations for each guest, authenticates with the Twilio account, and sends emails using the SendGrid API.