Remember that the purpose of the liner in these fall pumpkin centerpieces is to hold the wet florist foam and keep the water from seeping through the shell of the pumpkin.
The foam first needs to be soaked in water that has been treated with fresh flower food. Good designers don't skip this step, because treating your flowers correctly extends the life of every bloom.
Allow your foam to absorb water gradually, never forcing it below the surface of the water. To do so can create dry air pockets that can wilt flowers quickly.
After the Oasis is properly soaked, shave off the corners of the block of foam if necessary to fit into your liner.
You can see for this smaller pumpkin I've used approximately 1/3 brick of Oasis foam. The MaxLife foam holds a great deal of water, but must be replenished on a regular basis as the flowers will drink a good deal each day. Do not allow the foam to go dry.
Using a longer, smooth knife, cut down the portion that extends above the rim of the pumpkin, leaving about an inch above.
Never cut the Oasis foam down level with the container. This doesn't allow for lateral insertion of greenery and flower stems. This would cause the flowers to stick straight up, giving an unnatural look from the sides of the arrangement.