To send an SMS, you go to the Seller Leads app from the Operations workspace and select the lead you want to send the SMS to. Make sure that the Contact related to that lead has a valid phone number so we won't have any issues.
On the right side of the screen, you have the Activity section which has a comment bar at the bottom. In order to send an SMS, you need to type in the comment bar "text:" followed by the actual text body you want to be sent and hit Enter. After that, you will see in the Activity section the Communications item that has been created for this SMS. Also, in the field "SMS Communications" you will see every text that has been sent and received for this lead.
When the Seller texts you back, you will receive a mention to that specific Seller Lead, letting you know that a new SMS has been received, another Communications item will be created for the incoming text and also, the field Unopened Communication will change it's value into Yes (to easily see the leads with the field Unopened Communication = Yes, you have a View in the Seller Leads app called Unprocessed Leads).
If you want to have this feature implemented into your REIvolution you need to have smrtPhone integrated. You can request the integration from your REIvolution Platform by going to Phone System (see image below) and clicking on the Set up smrtPhone button or by visiting the Add-on Marketplace.