It couldn't be any easier. I put some ice in a ziplock bag and gave it to August. It was a brilliant opportunity to talk about things feeling cold - we'd already talked about 'hot' when he was super interested in my cup of tea this morning!
As usual he enjoyed scrunching and mouthing the bag - with the added bonus that the ice probably soothed his little teething gums more than any other sensory bag. I know that when it melts he'll enjoy it just as much - it's surprising how interesting a bag of water is to a baby!
Lovely language opportunities with this - I modelled the word 'cold' every time he was touching the ice specifically. I also touched it myself and modelled an over the top reaction to being cold - ''Wow the ice is cold! It's made my fingers very cold! I need to put my coat on!''. Later I took him to the fridge/freezer and opened the door to talk about the cold feeling in there. I showed him where the ice came from and we added a piece to the bag.