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 redirected to a checkpoint page and then from that page you should have to get the verification code on your e-mail/sms and then complete the checkpoint to get logged in.
Account Safety: When you login to IG Flash, your account will be logged in to our server once for generating your session cookies and that's it. We never misuse your account for spamming and any other activity on your behalf. Don't worry about your privacy, your account is 100% safe with us.