I start them in pretty basic gear.
My equipment to make starters:
Saucepan, stovetop, aluminium foil, thermometer, funnel, bung, airlock, 2 litre flagon. I used to use a 2 Litre PET bottle but the flagon is more durable and easier to clean.
It’sa two step process for me. First stage is to drain as much beer as possible from 2 750ml bottles of Coopers Pale Ale, while leaving as much yeast as possible. That’s actually the tricky part I think. Helps to start with the freshest possible beer, store it upright in the fridge a few days so the yeast is as settled as can be. Have a couple of jugs ready so you only have to tilt the bottles once to decant.
Then I tip dregs from one bottle into the other, via sanitised funnel.
I make a little wort of 400ml water plus 40g dried malt extract (or a little less), and boil it for long enough to sanitise. Cover with foil and cool it to below 30°.
Tip it into the beer bottle with the dregs.
Bung it up however suits you. I cover with foil first and shake it all up so it’s foamy, then usually put a drilled bung and airlock on it. Give it a bit of a shake from time to time if you like.
Once you can see it’s come to life and had a bit of a go and is maybe settling down, it’s time for stage 2, which is similar process except with a litre of wort and a 2 litre flagon to hold it.
Maintain sanitary practice and keep it at a suitable temperature throughout.
PET bottles in appropriate sizes are also perfect for this. They even have airtight lids so you can just tighten the lid when you want to shake them up, and loosen it a crack so they can vent the gas .
Or you can buy an Erlenmeyer flask and a stir plate and do it that way. Might be simpler and more predictable but costs a few more bucks. I’ve just always done it my way and can’t be bothered changing
This might be more coherent/easier to follow than my ramblings
https://brewsrq.com/blogs/you-can-brew-it/yeast-starters-the-shaken-not-stirred-method#:~:text=It's%20very%20passive%20after%20the,won't%20oxidize%20the%20starter.