Poor Penny. After her lovely day on Thursday cornering a rat and eating her bone, yesterday she snaffled a piece of plastic from under some meat and had to go to the vet so they could induce her to vomit it up. She's okay now, but was pretty unhappy when she came home.
We do have some washing soda crystals on hand to provoke vomiting in a case of poisoning, but on reading this veterinary advice, I'm sure glad our own vet was open and could do it for us.
For one thing, our vet had to administer a second medication to stop her continuing to vomit, and that would have been impossible for us to do at home.
The above link has some useful advice. For one thing, it's necessary in a case of poisoning to assess whether you might do more harm than good to induce vomiting if the poisonous substance is corrosive.
Secondly, there's some good advice about using dishwashing liquid to wash off a substance on her skin. We had to do that some years ago when she had a bad reaction to Advantix, a topical tick medication.