under ~1.040), you do not need to make a starter, although it would not hurt anything. If you are using dry yeast, you also do not need to make a starter, as dry yeast sachets contain, on average, a higher cell density than the liquid pitches of yeast (better rehydrate those dry yeast packets, if you are not already).
How long can beer sit before adding yeast?
Yep, no big deal to go as long as 12 hours. It’s better to wait 12 hours and pitch at the proper temp (below 70 for most ales) than to rush pitch into a warm wort anyway.
How do you use a beer yeast starter?
Add enough water to the boil vessel (dry malt already added) to reach the target starter volume. Add about 1/4 teaspoon of yeast nutrient to the boil vessel. You can use slight less for starters under 1-2 L and slightly more for ones larger. Bring to a gentle boil for about 15 minutes.