I'm not sure I made it clear that you're not OFFERING the food to her, you are actually making her eat it.
To do this, you'll need to gently grasp her upper jaw just behind the whiskers and under the checkbones with one hand, while aiming the syringe across her mouth with the other. It's not easy to do the first few times, but once you both get the hang of it, it goes pretty smoothly. (Hence the possible need to wrap kitty in towel first.)
For now offer her anything at all tempting as food first before force feeding her. mine devoloped a liking for roast beef that she'd never shown before. Sardines, anchovies, tuna, hamburger, lunch meat, leftovers - anything that you think may pique her interest. After feeding her a can of food slurry a day like this for a while, she may decide on her own to start eating regularly again. The main thing is to NOT let her go into starvation mode, that can cause her liver to overload complicating things for her and you.