Before you do the things in this article, make sure you've done the things in this other article.
Once you've successfully integrated SMS with the HubSpot conversations inbox (that's what that other article shows you how to do by the way), you can start sending SMS messages to contacts from the inbox. And if we did our job properly, you should see replies in there too.
In the main menu, go to Conversations and select Inbox:
Click Compose in the bottom left:
In the pop-up, search for the contact you want to send a message to and choose their Email to SMS (E2S) address:
Remember, if you haven't already done the stuff in this article, you won't be able to complete this step.
Now you just need to write your message and click send:
Your outbound message will appear in the Sent folder until a reply is received, then it'll move to the general inbox: