could I not just put something on a digestive biscuit
No.
You can either use a syringe or a dosing gun, or a special syringe with a dosing nozzle from Cox Agri.
It's not difficult but as with many things it's all about technique and confidence.
There is a gap in the side of a sheep's mouth, between the front and back teeth. Just behind that there is a lump in the tongue. You want to have the sheep's head horizontal or slightly above the horizontal and get the dose over/past that lump on the tongue - it can then only go one way.
If you use a syringe you have to tilt the head up (not too high but above the horizontal), lie the syringe along the tongue pointing at the back of the throat, push it in far enough to be sure it's past the lump in the tongue, and press. There is a risk, especially if you are using a large-ish syringe with sharp shoulders, that you may scrape the roof of the mouth, so take care.
If you have a drenching gun or nozzle, it is much easier. The nozzle is softly rounded so less likely to cause damage even if you do bump the roof of the mouth or the back of the throat. The nozzle is shaped so that you can insert it between the front and back teeth, in that gap, and it bends so that the dose is delivered past that lump in the tongue.
There's a good document The Eblex Parasite Control Guide. You can download it and other publications from their website
here.