Here is a simple tutorial. I'll do it with pictures.
Go down to the create tab. Click on it.
Click "Start from scratch."
Your screen should look something like this.
Click the plus button at the bottom. A menu like this should pop up.
Choose the square, drag it to the place u desire.
Click "add code" and add the "when game starts"
Then, u should get a huge choice of blocks. Scroll through, until u find drawing, the purple
Add a draw trail block. For the color or the first bubble, pick random.
For the second bubble, or the width bubble, choose 10.
Next, go inside the draw trail block, and scroll all the way to the control blocks, or the blue ones.
Then, add a repeat __ times block. In the bubble, put 4.
Now, add a move forward block. We want to move forward 100, so choose 100.
Now step twelve, I don't have a picture of. So basically, ur making it turn. Add a turn __ degrees book. In the bubble, put 90. Then play it.
Well, I hope this helped!