1. Signup Clickatell.com, buy credits and set up a HTTP api or get an HTTP GET api from any sms gateway.
2. Upload files to your whmcs installation directory.
3. Go to admin area->setup->custom client fields.
4. Create a text box field(tick required field and tick show on order form), this field is for your clients' mobile phone. Field name example: Mobile Phone
5. Create a dropdown field(tick required field and tick show on order form), type Yes,No without quotation marks into field options or type them in your own language. Field name example: Receive SMS notifications?
6. Go to utilities->addon modules->sms addon, enter your licence key and click install.
7. Enter all settings properly.
8. If you can see how many credits you have on module mainpage(for clickatell), you're ready to go :) If you get an error, check your API and module settings.