ohhhh well I would put the syringe near her mouth and gently push and if she doesn't give in to that take your thumb and with the pad gently push her bottom lip open and stick the tip of the syringe in, if that doesn't work then you probs need two people because one should be holding her and keep her still while the other trys to push at her bottom lip
I have done this before , just apply gentle pressure on the skin under the mouth of the chameleon and have the syringe ready. They don't like to keep there mouth open long so be ready, and slowly feed her.
"I thought this would be a great place to give people an idea of anatomy for future reference.
Unlike in mammals where the windpipe and esophagus are basically very close to each other in the far back of the mouth, chameleons (and most reptiles) have their windpipe at the bottom middle of the mouth and the throat is much farther back. The blue arrow points to the esophagus where the food goes. The red arrow points to the glottis, or opening of the trachea where only air should go. (in this pic the glottis is closed) That tube structure that leads to it is the trachea itself. The mouth has kind of a scoop shape. So the point is, when you squirt water into the mouth it will naturally pool at the bottom unless it is swallowed immediately. And this is bad because the windpipe is right there on the bottom of the mouth, which makes it very easy for them to aspirate! This is especially true if they are stressed and hissing, because then that glottis is already open (to hiss out air) and water can go straight down into it so easily."