Mother-in-Law....you mention "grocery bag"...I do hope that is a PAPER bag.

When plants are put into a confined space, such as a bag, there is moisture within the plant and it must be allowed to exit...confined in plastic, the bag doesn't breathe, doesn't let the moisture escape...mildew and rot has a chance then to start.....ending in the plant rotting.

Paper bags...left open...are best. Paper breathes.

There are 3 accepted ways to winter geraniums.
The first way is to keep them going, as they were in the garden, by placing into the sunniest window you can find....south and west are the more ideal; east will do in a pinch....north will not do.

Cut the plant back as you choose...this enables the plant to survive better because the sun is losing its value...and it doesn't return high enough to promote good health in plants until mid February/March.
Water as the plant needs it....but do sparingly. The plant will not grow in this stage...so it needs much less watering and no fertilizer. Feed only a growing plant.

The second way is to take as many cuttings from the parent plant and begin new plants that way.

The third way....the way I prefer..(I have 11 geraniums...7 of them are 8 years old)...
and this is done by having a coldcellar...(not a COLD cellar)...but a coldcellar...a rootcellar if you like.
This part of my basement is not heated, but retains sufficient less cold to keep the room in a constant temperature around 40 F. This is ideal to keep plants over winter by not letting them retain moisture.
The plant in this method is allowed to go completely dry....no sprinkling, no heat, no light...just let it go dry. Hang them up....upside down...(that is the easiest way to hang them...put string or cord through the roots)
This will create some debris though...so you might put something below them to catch the debris.

Geranium is one of a special breed of plants that can be allowed to dry up...completely.
Then when sun values return in mid February/March....they come out of their stupor to return to their old selves, their old bloom....as big or better than ever.

All that is required is keeping them in the cool temperature. They must not freeze..a frozen geranium is a dead geranium. They cannot, unfortunately, keep in an environment where the temperatures are above 45 - 50 F.

Then in February/March....they are brought out, cut back, given new fresh potting soil, water...put in good sun...and they will re-grow.
When new leaves form....might take 3 or 4 weeks...begin to feed 1/4 rate 20/20/20....and increase as the plant foliates out.

The plant will produce new foliage....and maybe in about 6 - 7 weeks, flower buds will be sighted. But don't panic if they are slower...they will respond when light values are higher.

In the window, turn the plant 1/4 turn every day to ensure equal sun for all parts.

When you water initially, do not water again until new leaves form. Then water as needed....but always, water to drainage....and dump the excess. Never let the plant sit in drainage water for longer than 20 minutes or so.

That's it....that is the way to bring geraniums back year after year after year.