The 'what' can honestly be anything and they 'how should' is all down to what you want to do.
A suggestion though... If you've got the kind of food that could be eaten with just a fork, then you can try putting the slave in a straitjacket or maybe even just cuffing his/her hands behind the back and then force it to eat off the plate like an animal.
I've seen some sites where the submissive has liquid enema'd into their butt which they then drink.
I'm not sure what the health issues with that are though, but I'm sure there are some.
You could also try with runny foods?