I'm going to be completely honest with you: I do not think that this writing assignment should be limited to a single paragraph, as the prompt indicates. Descriptive writing specifically writes to a reader's five senses; describing what that sand pit looks like, feels like, tastes like, sounds like, and smells like in a single paragraph won't do any of those specific senses justice.
Part of writing a descriptive paragraph involves using clear and concise language. That doesn't mean make the descriptions short, but it means stay on topic and avoid vague language. Don't tell your reader that the sand feels rough. Tell your reader that the sand has a gritty sandpaper like feel against your bare feet. Write about how it fills in the gaps between your toes and rests on the webbing of your toes. Explain how when you put too much pressure on any one point of the foot that the tiny sediment crystals dig in and cause momentary snaps of pain, and explain how you occasionally think that certain grains might break the skin and cause tiny rivulets of blood to trickle out of your feet.
The goal of the descriptive essay is to paint a picture for your reader, so using similes and metaphors is going to be a big part of your paragraph. The comparisons give your reader a way to relate something unknown to something that they perhaps have experience with. For example, if the sand pit is steep and deep, then climbing up and out of it is going to force you to look up and see only blue sky. You are either staring at a wall of sand, or you are staring at blue skies. Perhaps you relate that to being in a small plane upon takeoff. Climbing up out of the sand pit was like climbing off of the runway in a small Cessna aircraft. One instant you are looking at land, and the next you are looking at clear blue skies.
Try to brainstorm what the weather is like. I know your prompt says that the weather is a hot summer day, but is it windy too? If it is, then perhaps there is some blowing sand. Obviously that has a feel to it, but there is also a taste to it. Those fine grains of sand are easily carried by the wind and get in your mouth. They coat your teeth, and no amount of tongue scrubbing can get rid of that taste. If the wind is not blowing, then you are still going to have taste involved. The lack of wind means that you are probably sweating, and that sweat will drip down and come to rest on your upper lip. Describe the salty taste of your sweat as you struggle out of the sand pit.
If you are in the middle of nowhere, you are likely surprised at the lack of sounds. Write about the silence, but don't focus too much on the silence because you are only being surprised by the lack of human noises. What desert sounds do you hear? With each step, you disturb the sand, and it cascades down the steep pit. Describe that sound. Is it crunchy? Does it sound like water in a weird and unique way? Perhaps the silence is so extreme that you finally notice the desert hawks making their cries in the distant skies. These are all details that help build the image of that steep desert pit experience for your reader.