Cut the Pumpkin so that it is possible to put a glass or cup or similar vessel inside and to be able to remove it to fill it and to empty it. A door/hatch in the back is a good way to do this.
Make sure the point where you want the fog to come out is below the top of the vessel. The fog will start to come out of the hole at the lowest point, so make sure there are no escape points lower than the one you want.
Fill the vessel with hot water (not too hot or boiling for safety reasons) and when ready, add dry ice. If it is 16 mm dry ice sticks 2 or 3 sticks should be enough. If it is 3 mm something like a heaped desert spoonful would be a good start. Close the hatch to stop the fog coming out there.
If the fog effect is not as much as you want, either you have not used enough ice or the water is too cold. Adjust accordingly. You will find that a small amount of hot water cools very quickly with the ice, so the bigger the vessel the longer and better the effect. If the effect stops and there is still ice in the water, throw it away and start again. Do not add ice to cold water, it will not have much effect.
Another good show is to use a cauldron. Here you will have the chance to have much more hot water, and throwing in a good quantity of ice will cause the fog effect to flood across a table and drop to the floor.
Add effect to the fog with coloured lights.
For spooky food and drinks put the plates or glasses on a tray which can hold the hot water and add dry ice just before serving. BE CAREFUL TO ENSURE THAT NO DRY ICE COMES INTO CONTACT WITH THE FOOD OR DRINK. KEEP DRY ICE AWAY FROM CHILDREN AND DO NOT TOUCH IT WITH BARE HANDS. IT CAN CAUSE BURNS.
DO NOT CREATE DRY ICE FOG IN AN UNVENTILATED AREA OR AN ENCLOSED SPACE.