To add one contact to a campaign, you can go to the contact profile and Click on the Campaign tab, then click on the Add to Campaign button.
Then, you select the campaign you want to add the person to. You can see the campaign summary as well all the actions to be sent in the campaign preview, you can also select the action you want to start the campaign in.
To send the campaign you can send it right away or you can schedule it for the future.
Once you send the campaign you'll be able to see it in the contact profile. Click on the icon next to the campaign name to open it and see all the actions for the campaign and their status.
In case you need to edit a scheduled message from the campaign you can edit it from the contact profile.
To edit the step you can click on the scheduled action preview:
When you click on the Preview the system will show you the original scheduled message, when you click on the Update option you'll be able to edit the text:
Once you edit the message you can Submit it and the message will be changes for that contact who is on the campaign.