just pick a day when the weather is said to be nice and pack your bag or what ever you want to bring (for me it was a bottle of water, 2 sandwiches about half a dozen snack bars and an apple or two and i got home with some still left and hadn't spent a penny of the spare tenner i brought with me
plot a route on your garmin or on a map or what ever and just go to bed early to get up early and off you go, after about an hour or so i find i just keep on pushing.
to minimise risk of bonking or running out of energy and just dying i made sure i stopped, even if just for a minute or two at the road side, every 20 miles roughly to have a sip of water and maybe a snack bar and then had a longer (maybe half hour or so) break for lunch
that was my first 100 this year on my old 13kg stell frame flat bar hybrid on 38mm tyres
oh and whilst i am on about imperial centuries i also want to do my first 100 on my new bike, longest so far in 88.3 miles and now i wish i had done the extra 11.7 to make the big one but i was just dead

Cheers Ed