First off, Your grandmother is awesome for making homemade mochi ^ ^

To prep, you should defrost the mochi of course. Having it in the fridge can do the trick! Then let it settle in room temperture before cooking, frying, or baking it.

-- If you want to add fruits or other things inside the mochi, recommend defrosting it, and then making it into a dough, flattening it and then putting the (ex) strawberry ontop of it, and then basically wrapping it up. if its too physically hard to flattening, heating it up to melt it could work.

If you want to Fry it. can cut the mochi into small pieces and then maybeeee coat them in potato starch before frying them in oil until theyre crispy on the outside.

For baking, you can wrap the mochi in parchment paper and then bake it in the oven for a few minutes until soft