First of all you need to sign up herehttp://smsglobal.com/signup/signup_page.php
After signing in, you will receive your username and password on your phone.
Now login in your SMS global Account with your provided Username and Password.
After loging in, you will see the welcome screen, just click on Send SMS to Number
Then remove the sender ID while sending to send a spoofed SMS after that enter the mobile number of both the victim and now you can send a spoofed SMS/send sms anynomously