Before you battle a legendary bird always have at least 30greatballs and don't forget to save before you attack.Use a sleep move like Hypnosis or sing to make it go to sleep. then,whittle down its HP but do not make it faint. If you do make it faint,turn your game off and then turn it back on.Once its HP is low throw greatballs till it is caught.If it is Lugia, use a masterball.
Here are their locations.
Red,Blue,and Yellow only
Moltres:Victory Road
Articuno:Seafoam Islands
Zapdos:Power Plant
Gold,Siver,and Crystal
Lugia:Whirl Islands
Ho-oh:Tin Tower |