I like enlarging the hole by marking the dimensions with a pencil or marker, and then, using a chisel and hammer, chip away at the edges until it is the size  I want. It is not important if the edges are smooth, or the shape is exact.

Place the glass in the pot. It can be placed in many different configurations to get different effects.

Note that I've only used a tiny amount of black glass. The black has a tendency to overwhelm the other colors.

Three pounds of glass will result in a melt approximately 11"  in diameter.

Do not apply kilnwash or shelf separator to the pot. Yes, glass will stick to the pot, but you can still use the pot again if you're using similar colors. If you put kilnwash on the pot, you run the risk of  getting particles of kilnwash in the melt.

Simple kiln setup. Paragon GL22AD kiln, 5" of vertical kiln posts. Mullite shelf with 6 thick coats of kiln wash, placed on 1/2" kiln posts. Place pot as centered as possible in the kiln. Make sure the glass does not touch the thermocouple in the roof, and the hole is clear of the supports. I'd recommend applying kilnwash to all kiln furniture, and make sure you have applied kilnwash to the firebrick bottom of the kiln itself in case there is an accident.

If you have a very tiny kiln, you can use very small flowerpots, or even just the small ceramic flower pot trays to hold the glass. If the distance between the pot and the shelf is very small, the glass might not swirl, but you can get some interesting patterns nonetheless.

This is what happens with a rectangular hole. The glass folds on itself like taffy. With a round hole it spirals in a circular pattern.

My recommended schedule is :

1000 dph to 1000F, hold 15 min.,

250 - 1100 hold 15

1000 dph to 1700, hold 90 min.,

AFAP to 1500F, hold 45 min

AFAP to 960F hold 60 min

200 dph to 750F

300 dph to 300F

off

The hold time can vary depending on how quickly the glass flows out of the pot. Once it is not flowing anymore, you can move on to the next step. If there is still a strand of glass flowing from the pot, you might want to hold longer if you are concerned that there might be a  stalactite. (See note below)

The slow ramp between 1000F and 1100F is to prevent cracking of the clay pot.

The hold at 1500F is only to even out the thickness of the melt (to about 1/4"), and allow bubbles to come to the surface, pop and smooth out. If there is a central "belly button" where the stalactite met the melt, it will help that smooth out also.

 (I didn't spend a lot of time taking this photo.)

Important: If you are going to look into the kiln at 1700F you must take certain precautions. Wear protective eyewear, and even better a face shield. Wear protective high temp gloves as you open the door to the kiln. Be aware that synthetic fabrics are dangerous at these temperatures. It is safer to be wearing cotton. Synthetics can burst into flame or melt causing burns. It is best to avoid opening the kiln when it is at 1700F.

Finished melt with rectangular hole.

Note: These melts will always have kiln wash (a.k.a. shelf primer, shelf separator, etc.) adherent to the back. This will need to be sandblasted off, or removed by some other method.

If you need more information, I'd recommend searching that board for information - you'll find lots of good tips.

Heres a link to one interesting, discussion.