You might not like this advice, but my opinion is that if you can't do quarantine right, don't do it at all.
To do it right you'd need another place to keep chickens that's far enough away from your current birds that dander won't blow from one bird to another. My quarantine area is 50 yards away from my chicken coop, with a large pole barn in between. If you can figure out a place that's far enough away, then here's how to quarantine properly:
1. Always feed the existing birds first, THEN go to the new birds.
2. Once you've been to the new birds, then a complete change of clothes and a shower and sanitizing your shoes is necessary before you go back to the existing birds.
3. Don't share ANYTHING between the groups--not dishes, waterers, feed bags, etc.
4. If you can, choose a potentially sacrificial chicken from your existing flock to live with the new birds. That's one of the easiest ways to see if the new birds are carriers for something but not showing symptoms.
5. Keep the two groups utterly separate for at least 30 days.
6. Always be aware that it might be your EXISTING birds that are asymptomatic disease carriers, and they might give something to the new birds.
All of this is far easier said than done, especially if you don't have a facility for it. Here is what I would do if I were you:
1. Buy chicks from a reputable hatchery. Then your disease risk is minimal. But, of course, you'd have to have a separate place to raise the younger birds away from the older birds, so that might be out for you.
2. If you really want to buy started birds, don't commit to buying over the phone. Go look at the facility*. See how it smells. Look at the person's other birds that they're keeping. Listen carefully for respiratory sounds. Pull back feathers and look for mites. DON'T meet someone somewhere to pick up the birds--go to their place.
3. If the place doesn't look or smell right to you, trust your gut, don't worry about being polite and leave ASAP. Run your car through the car wash on the way home, and shower and change clothes and sanitize your shoes before you handle your own birds.
4. If the place seemed OK to you gut, then bring the new birds home. Deworm them and treat for mites (some pour-on dewormers do both in one step).
5. Put them in a place that the existing flock can see them and hear them but not peck at them, like a large dog crate. Let them live in the dog crate for a week or so, then let them out to range with your girls during the day. Let them go back to their crate to roost for a couple of days. Then, one day, let them out to range and take away the crate. There may be some pecking and chasing, but it should be OK.
*one caveat--I'm fairly conscious of biosecurity, and if you were buying birds from me and I knew you already had chickens, I would have pulled the birds you were coming to buy out ahead of time and put them in a carrier and kept them in my pole barn, away from the hen house. I'll let people stand outside my pasture fence or look through my chicken house windows, but I don't let buyers inside my hen house. I worry about buyers who already have chickens at home bringing disease with them.
