To add the contacts to the sequence.
1. Please go to contacts, and select all the contacts that you want to add to the sequence. Please remember to select the blue link that says "Select all subscribers" as shown in the screenshot below:
2. Click on bulk action, select Add to Sequence and select the sequence
Note: Please don't make any changes to the sequence once the contacts are added to it. However, if you want to make changes please remove the contacts from the sequence, make the changes to the sequence and add the contacts again to the sequence.