It’s probably more art than science. When I’ve used the cream I just squirt about a centimetre out and get it into them however I can. Try injecting it into a piece of bread or giving it to her in a dark room so she struggles less.
If she is acting normal, poops are normal, and her crop is emptying, I would put her back out with the others. Continue the treatment for 7 days, and keep checking her crop in the mornings to make sure it’s empty.