Hi Remy.
There's no set recipe. I use a 50ml syringe with about a tablespoon of glucose and a tiny pinch of salt with the cereal component - I've not tried readybrek but it sounds ideal (warm) and will help the rumen to keep working.
If you have a catheter-tip syringe, such as those for using when tubing a lamb, that is ideal, rather than a tiny 5 ml syringe. The catheter tip is about 1 1/2 inches long so you can put it in from the side, between the front and back teeth, and get the food nicely back on the tongue. Aim to get at least 50mls down at each feed, very slowly, checking she swallows between each few mls.
Something worth trying when she shows an interest is digestive biscuits - they have sugar, salt and cereal, plus a good crunch which sheep love. You may have to push tiny pieces into the side of the mouth on top of the tongue at first, but not until she is swallowing the readybrek happily. Once she gets used to the biscuits she will be addicted for life.
Then when she is showing signs of perking up add in handfuls of grass, willow leaves and so on to get her back to eating properly.
Also make sure she has enough water - this can be given by syringe too if she doesn't drink from a bucket.
Keep her indoors for nursing until you are sure she can manage outside. If she is 'down' then stand her up at least every two hours. Hold her up while she passes urine and droppings, and try to get her to take her own weight. If she flops when you lay her down, prop her up with hay bales.
With good nursing a sheep can come back from a severe illness to full health.