Just click on the "Login with Instagram" button and enter your Instagram credentials in the form and you'll be logged in to IG Flash.
Location Checkpoint (First Time Login): If you're using IG Flash for the first time then your account will get a location checkpoint, you'll be asked to complete the checkpoint challenge by getting an OTP on your mobile number or e-mail associated to your Instagram account, once you complete the checkpoint challenge, you'll be logged in to IG Flash.
Account Safety: When you login to IG Flash, your account will be logged in to our server once for generating your session cookie and that's it. We never misuse your account for spamming and any activity on your behalf. We don't also save your credentials the only thing we save is your session cookie and nothing else. Don't worry about your privacy, your account is 100% safe with us.