i wouldn't try a 24IF for your first one. i've been doing PB since august and just last week had my very first one! i did 12, 14, 16, 18 & 20hr fasts prior to it. it took me so long because well i don't really plan for these things. if i was to make IF a priority i still wouldn't do it for myself no more than 1x a week.
as many have said, i'd have the end of your IF or the hardest part be when you go to sleep (ending overnight). i did last meal at 7pm the night before (very heavy in fats and low carb) and i was able to make it through the day quite nicely drinking lots of water and green tea. as for the first meal post IF, eat slowly! be conscious of yourself and don't overeat. listen to your body. maybe soup first then start into the solids, but chew them