Climb in and start shooting you never know your luck!
Garden birds shouldn't be too bothered if you're setting the hide up next to a feeder for example.
If however your target is a species a little less common or close to a nest site, then things are a little more tricky...It may take a few days for these birds to get comfortable with the hide.
A pretty common method is the use of a decoy...Have someone walk to the hide with you and a few minutes after you've set up and are ready to shoot, your decoy walks away from the hide, hopefully fooling the birds into thinking there is no human presence.