You can cut out the larger hole with a tin snips or you could use an oscillating multi tool equipped with a metal cutting blade.
Cutting hole in gutter for downpipe.
Insert the drop outlet in the hole and fasten the new downspout with sheet metal screws.
Step 2 drill four holes.
How to cut and install a gutter outlet.
How to cut a downspout into a gutter.
This one came with holes in it for rivets.
Step 1 find the right location.
Step 1 is to draw the shape of the hole and drill s few holes around that shape.
Drill a hole in the bottom of the gutter at the 4 inch mark with a 1 inch hole saw attachment to the drill.
Here you can see the shape of the downspout flange i had.
Step 3 cut the tracing for the connector with a pair of sheet metal cutters.
Make sure to seal the drop outlet to the gutter with seam sealer.
Step 3 file the edges and apply sealant.
Then i used a fine tooth metal cutting blade in my jig saw.