For a start - the most important thing, as people have said, is dosage. It's normally shown as 'xx ug / hour', though they should also have a total dose. If I remember correctly, 100ug = 16.8mg, 50ug = 8.4mg, and so on.
An approximate way to compare the dose to oxy 1.5mg = 80mg oxy. HOWEVER, it is very important to measure your own limits - as people above me have said, fentanyl is one of the more dangerous opis. There have been many cases of peoples hearts simply stopping - not even just the regular OD where you stop breathing.
I would start by chewing about 0.25mg worth of a patch. To measure the dose, I usually like to divide the patch into squares. I normally had 100ug / 16.8mg patches, so I would just cut it into quarters, and then those pieces into quarters, to get 16 pieces of about 1mg each. Work this out based on your own patches. Once you've got the dose figured out, just chew on them and hold under tongue for about half an hour. After half an hour or an hour, depending on how you're feeling, try another 0.25mg (or even smaller increments.)