A recipe calls for buttermilk but I don’t want to go to the store just for that. Can I make my own
Yes you can, if you have full fat cream.
The process is straightforward but a little time-consuming, and it will be much faster to just go buy a bottle of buttermilk.
Buttermilk is just the residual liquid of butter-making (see here: you have your cream, better if cultured, and you whip it (a food processor will do it) until the solids thicken into butter.
The liquid that remains is buttermilk