I have always had good luck slipping chicks under a broody hen. There is a trick to it, though.
When you get the chicks, make sure they all get a drink of water.
This sounds heartless but chicks and hen is a two way street. The chicks have got to want the mama, as much as the mama wants the chicks. The way you do it, is in the late afternoon, near dark let them get chilled. Chilled chicks peep madly, that is what you want.
Take the chicks down to the coop, cheeping out side of the coop. It should be completely dark now. Let them set out there for a few minutes so she can hear them. Then with minimal light, as close to dark as you can get, wearing gloves stick the chicks under her. If they are cold, they will burrow in like ticks. That is what you want... and it should get silent. The broody hen will change her cluck, and start talking to them.
THEN LEAVE them alone. A lot of people trying to be nice, keep adjusting things, using enough light so they can see if is working, wind up upsetting the broody hen, and then think she is a bad hen because she doesn't take them.
Mrs K