I assume you have no equipment to sieve so you can seperate as much by hand as possible then use water to seperate Have the bulk mixed material in an elevated position and run water thru it so it drains with all the sand/ finer material into a lower container repeat this till you get the result you want
I would get a metal mesh screen, put together a frame out of 2x4s and stretch the mesh over the frame, holding it together with screws through the mesh. Stretch it over the top of the 2x4s and around to the bottom. That will hold it in better. Then use that as a filter to sift the sand through (from the top, or the weight of the sand/gravel will just tear the screen off eventually). The best way to do it is to set the screen across the top of a wheelbarrow. Put a couple shovels full of the sand/gravel across the top of the screen, and shake back and forth. The sand goes into the wheelbarrow and you can dump the gravel off the top of the screen when it's full. I did this with a 3/8-inch steel mesh screen to filter a bunch of gravel out from the soil where I was going to plant some grass. The previous owner had dumped a lot of gravel in the dirt while building a retaining wall, making it impossible to dig through the dirt properly. It took a while, but I got rid of it all.