What happens when you order
You put your username in the box and press the button. The next screen opens with it already filled in. You choose how many followers you want, pay, and the order starts after that.
Where to find it
Open your profile. The name at the very top of the screen is your username. Leave out the @ if you like.
Check this first
Switch your account to public before you start. You can make it private again once everything has landed.
Why order Instagram followers
There's a setting called "Flag for review" under Settings, Follow and invite friends. When it's on, Instagram holds back some new followers while it checks them. Turn it off first so nothing gets stuck. New followers trickle in over a while rather than in one jump, which looks more natural anyway.