I've started doing something new in my home brewery. In previous batches I only proofed my yeast. I was typically using an emulsified dry yeast. To get the yeast active for their fermenting duties, I'd pour the packet into a quarter cup of warm water in a sterilized measuring cup. Doing that before starting to boil the wort gives the yeast almost an hour to get active. Even older yeast can yield good results with this simple method.
In my latest batch, however, I switched to starting my yeast. The day before brew day, we'll call it starting day, I boiled two cups of water with a quarter cup of malt extract then chilled that pint of wort and pitched my yeast into it in a sterilized flask. Boil to sterilize the water and dissolve the sugar then chill down to a temperature the yeast are happy with; same as you do on brew day. For the sugar, malt or malt extract matched to what the yeast will be working in is best but even regular table sugar will work. Overnight, the yeast multiplied and grew strong in their flask and I pitched in the entire solution into my wort for a quick and ready start to the fermentation cycle. Some sources recommend letting the yeast settle into the bottom of the flask and pouring off most of the water before pitching it into the wort. It's the brewer's choice on what works best for them.
Started yeast also opens up a new opportunity for the home brewer; yeast wrangling. Instead of pitching the entire starter into the wort, double the recipe to make a quart of starter but only pitch a pint into the fermenter, saving the remainder in your refrigerator. Then every starting day, bring out your corral of yeast, add warm wort to bring the volume and activity back up, and proceed like before, reserving half of the herd for next time. When you're wrangling yeast, you do have to keep an eye on your herd to prevent infections and keep them well fed so they're ready every brew day. Actually, keep your nose on the herd; you'll smell off flavors developing in the starter if an infection takes hold. Keep you equipment as clean as you would on brewing and bottling days and you won't have any trouble. Happy brewing!
Please drink responsibly.