How can I measure / weigh really small quantities?

Simplest solution: Buy a more sensitive scale. There's plenty around that can measure grams.

If that's not an option, you can sort of just about get it quite, but not completely wrong by using measuring spoons:

A full teaspoon with something in it is usually around 5 grams. A quarter teaspoon would be 1.25 grams, if you happen to have a 1/8th measuring spoon, it would be around 0,6 grams.

I'm not very proficient in baking (or not at all, really), but what I notice (or think I noticed) is that correct measurements in baking are more important than they are in cooking. So the best advice would still be (since you're baking a focaccia) to buy a more sensitive scale.

Convert it to volume. You can easily google the density of yeast or olive oil. When I did it for yeast I found that there's about 2.8g per tsp. So, a heaping 1/4 tsp is about 1g. I also found that 5g of olive oil is about 1.1 tsp.

The site I found is here: http://convert-to.com/

You can measure a power of two, and then divide. For instance, you measure 15 grams, which is almost 16, and then you halve it, halve one of the halfs, etc., four times: you end up with one gram. Or measure 30 (almost 32) and halve five times.

A decimal gram scale is a lot more affordable then many people think. I have this one (https://www.amazon.com/TREE-KHR-3001-Kitchen-Scale/dp/B01HKK4GYS) and it is great. It's accurate to .1 gram and measures up to 3 kg.

Another option that's used often in analytical laboratories for the same reason: stock solutions!

Since you want to use fairly cheap ingredients, you can create a "stock solution" (in water¹) and measure that with much more accuracy.

In your case, you can measure 10g each of salt & yeast, mix them with 80g of water to make 100g of a stock solution² containing 10% by weight salt and 10% b.w. yeast. You can then measure 10g of this with fair accuracy and add it to your dough, effectively adding 1g each of salt and yeast.

The main downside of this method is that you are wasting salt & yeast to make the stock, but seeing as they're both fairly cheap I don't see it as a dealbreaker. The other point of attention is that you're adding a bit of extra bulk ingredient (8g of water in the above example), which you might need to take into account at some other point.

Footnotes:
1: You can substitute water for flour, or any other bulk ingredient, however solid ingredients require very careful & thorough mixing for the above method to work
2: More accurately it's a dispersion as yeast doesn't really dissolve