There are lots of sites on the web about this - the 5:2 diet. A friend lent me her book on it. If you google it you will find loads of recipes and ideas.
For the women it is 500 calories per day and the men 600 calories. I cannot do the two days back-to-back so split them up and move them around according to my diary.
I am pretty lazy and don't want to fuss about calorie-counting so usually stick to the same things each time. I have variations on a scrambled egg on toast for breakfast, two crispbreads and very low fat cream cheese trianges with a smidge of marmite for lunch, plus half a banana; then a dry-fried chicken breast with asparagus and spinach for supper plus the rest of the banana. (A tip here - if you cut a banana in half through the skin the remaining half will keep for days.) I am about to try the zero noodles form Holland & Barratt.
I drink lots of water and tea with a tiny drop of milk. I don't take sugar but if I have a green tea then I add 1/2 tsp xylitol. I don't drink black coffee and normally have this really creamy so I just refrain on fast days.
To start with my fast days ended with my feeling famished and having a headache but this has now ceased and I'm fine. The other 5 days I find that I do not eat so much out of choice - I still eat cake and cream.
Wishing you success.
My hypothyroidism is nearly optimally treated on 120mcg T3 so I'm feeling fine.