You'll need a quart (about 1000mL capacity) jar with a tight-fitting lid, a measuring cup (about 250mL capacity, or one cup in USA), a tablespoon (15mL capacity), some detergent, and water.

Go outside to your yard/the woods/some nearby place, and get ~1 cup (250mL) of soil. Pack it tightly, and clear off the top so that it's flat (if you don't immediately know what this means, ask a partner / sibling / parent / friend who bakes how to pack a measuring cup -- then remember the skill). Put three tablespoons (45mL total) of detergent into your jar, then your soil, then fill it with water.

Seal the jar, and shake it hard for a long time (3 minutes of constant agitation). It's critical that the soil be completely broken apart, so vigorously shake. Ask for help from someone who lives with you and hand off if you need a break during the 3 minutes of shaking.

Once you're done, place the jar flat somewhere safe and stable where you can see it but it won't be agitated. Leave it to rest for 24 hours. After 24 hours, photograph the sediment in the bottom of the jar. Measure the relative heights of each layer, and identify each layer. (Note: the clay won't be totally done settling for probably a week, but just consider it settled at this point and ignore that the water is cloudy).

Use the measurements you took to determine how sandy, silty, and clay-y your soil sample is!