Select the route of sending sms either promotional or transnational
Click on the send custom sms block as shown in the image
After that you will reach at this position as in image.
Now click on the browse CSV. you can also get the sample of the particular CSV file by clicking on the link named as sample file. you will get all information about in which format you have to make the csv file to use this feature
Select the CSV file which you have made as followed
Select the fields by clicking at only once on then to insert into your message.
Either click send now button or you can also schedule the sms as per your needs.