You will need to towel him to do it. Keep him in one hand, brace his head with your finger and thumb so he´s not wriggling.
Not ideal to towel a bird, but for crop feeding and other treatments it can be necessary. It can also reduce the time it takes to do the treatment/feeding, ergo cause less stress.
I´m assuming you mean gavage feeding here? (Directly into the crop) Did they give you the tube? It´s disappointing they didn´t give you a visual demo, that really is the best way to learn something like this. A video is not the same as having a person show you. Perhaps call back and push for a demo?
If they are wanting you to syringe feed (where there is no tube going into the crop), then you can slowly give the food, super slowly so he can swallow it, if he stops licking/swallowing, you stop feeding, waitfor him to clear his beak before doing it again. You can´t force a feed or you will risk aspiration.
I don´t have more specific tips for your bird as I have only fed chicks. Hopefully other members can offer their insights.