1 - The first step is to go to a Email Spoofer Website for that you can use https://emkei.cz/
2 - Enter the details it ask you, that will be name, email sender, receiver, subject and text
3 - If you using this for phishing just put the "From e-mail" to the company/person you are trying to impersonate
4 - Just click Send, with the site i showed you sometimes it goes to spam, but there is other options to make it go to inbox.