There are a lot of different things to learn about using a box blade, but if I had to give you one, I would say that the angle (as controlled by the length of the top link) is key to controlling how the blade behaves. If the top link is short, the front blade will bite in deeply when you drive forward, and you will remove a lot of dirt off the surface and carry it forward. The blade will quickly fill up. If the top link is long, the front blade will not bit in at all; the box will rest on the back of the rear blade, and dirt will fall out of the box and be dragged smooth by the rear blade. Controlling a box blade's behavior is, in large part, a function of setting the length of the top link so that it has the right amount of bite--not too much and not too little.
Unlike a land plane, where you just make pass after pass over the road, or a grader blade, where you would make one pass to create a windrow of dirt, and another pass to spread it back out again, with a box blade, you must take the road in chunks. The blade will fill up with dirt or gravel, and then you will have to lift it, dump out the contents, then move forward slightly and start again. After having done this, you will end up with a bunch of speed-bump like piles of dirt in your road. Then you spread them out. Some people don't feel comfortable pushing with their three point, but if you are willing to push with your three point hitch, you can just back up to spread the piles back level. The same angle of top-link that makes the blade bite in when it's moving forward will make it slide over and spread the dirt when it's moving backward. If you are willing to push with your three-point, you can get a good rhythm where you pull forward to scrape up and gather the top inch or so of dirt until the box fills up, then you raise the box, pull forward, drop the box, and push backwards to spread and smooth that dirt over the section of road.
Alternatively, you can lengthen your top link and spread the dirt going forward, but in this case, you will have to scrape up all the piles first, then change the top-link length, then spread them back out again. If you have a FEL, I find it is also sometimes useful for finishing up the spreading, by back-dragging.