Make a quick painting from one of the following photos in which all the edges are soft.
You will need to get the paper wet enough to stay wet long enough to put down the lights, the middle values and the darks. I recommend wetting both sides of the paper, either by working clear water into the fibers with a big brush or by running the paper under a faucet for 20 seconds, or so on each side. You could also soak the sheet in the tub for a few minutes, but don't use hot water. It wreaks havoc on the sizing. Drain off any excess water by holding the paper up by one corner till it stops dripping. It should be uniformly shiny.
As much as possible, think of the wetness of the paper as your water supply. Stay out of the bucket unless you really need more water. It helps to keep track of how wet the brush is compared to the paper.
As long as the paper remains wet your marks will have soft edges. The thicker the paint is on the brush, the more defined your marks will be, so there will be some variation in how soft the edges are. It is much easier to control the amount of feathering your edges have by adjusting the thickness of the paint than by waiting for the paper to be just the right wetness.
Looking at just the blue strokes in this sketch reveals a range of soft edges at work. The stroke in the top center, for example, has a more defined edge on its left side than on its bottom. Both edges are soft, but one is softer. Look over the sketch to see if there are any edges that you would call hard.
When you are working on the homework exercise, be vigilant regarding hard edges. As soon as you see one, stop painting. Dry the paper thoroughly and re-wet the area you want to work on. Be efficient in re-wetting so you don't disturb paint that is already attached to the paper.
It's a challenge to paint an entirely soft-edged watercolor. If you get a few small hard edges that's close enough. The sky sketch, above, for example, would be a reasonable job.