Baking lots of bread - keeping a yeast starter
Making a starter. I suggest you make a sourdough starter but add yeast at the first stage. I would begin with 50g wholemeal flour, 50g water, and 1tsp instant yeast, and then feed it twice a day, maintaining equal weights of flour and water and discarding (or otherwise using) half the starter each time. Keep this going for at least five days.

Once you have an active starter it will survive unfed at fridge temperature for up to a week. Make sure to use a clean container and a clean fridge to avoid contamination with mould.

Incidentally, your starter might also pick up wild yeast. I would expect wild yeasts to be out-competed by the commercial yeast and for the yeast strain to remain stable, but this isn't guaranteed. Either way you get an active starter.

Checking for yeast activity.
You can get some idea whether your starter has active yeast by looking for bubbles on its surface or an increase in volume after feeding. But I've had sourdough starters that barely grew after feeding but could still raise a loaf.

Another quick way to check that your starter has active yeast is to fry a tablespoon of starter on a high heat. I find there is just enough time for the yeast to develop a matrix of bubbles in the top surface before it dies. If there is an acidic smell and no bubbles then I'm afraid the yeast is dead and you'll have to try again.

A much better way to gauge the activity of a starter is to use it to make a loaf. I expect you will find your starter has less activity than instant yeast (or active dry yeast) but you will still get good bread. In fact because your starter will pick up natural bacteria (like a true sourdough starter) you might even get better results.

Using your starter.
I typically give my dough a long first rise in the fridge: this allows for some variation in yeast activity since the whole process happens much more slowly. It also gives a deeper flavour. One sourdough rye starter I maintained for several years had relatively little yeast activity but was still able to raise a good loaf given 48 hours in the fridge.