Its easy... use your senses.
Listen for birds or suspisious movement in cover near you.
Walk slow through thick stuff... take a few steps, and wait, repeat.
Walk.... LOTS. Cover as much ground as you can.
Think like a bird... i.e. If you were a bird, where would you be? High noon... find shade... etc.
Spot and stalk sounds silly for birds... but it work. Glass a field edge from a few hundred yards away and sneak up to the birds you see.
And don't injure birds. You have to shoot to kill. If you injure a bird, without a dog... you might as well not have fired at all.
Don't get greedy when you hunt without a dog. I.e. if 5 birds bust up, keep yer eye on closest one, kill it and follow the bird to the ground. Never take your eyes off of it.
Often the hardest part of hunting without a dog, is finding the bird.