Pumpkin centerpieces are fairly easy to create. My first opening for my pumpkin centerpiece is roughly the size of my container, but I will have to do more trimming after the pumpkin is cleaned out.
Remember - it is better to have the opening too small - rather than too large. You can always trim away the excess until the fit is perfect.
You can discard the seeds and pulp, unless you'd like to toast the pumpkin seeds later.
Carve away the excess pulp and seeds off the pumpkin lid as well - you'll be using it in your flower design.