Here is the gun from the right side, then from the top. The final picture is of the projectile I use. (the projectile can be changed to your preference)

Step 1: Top Railing

In the first picture is a segment if the top railing. Next, build six more of them. The second and third picture are of the segment that will go in front of the trigger assembly. The fourth picture is the rod that will go on the end of the gun to hold the rubber band in place. Then follow the pictures in the process of assembly. The final picture is what it should end up looking like.

Step 2: Bottom Railing

build the railing in the first two pictures, then build a second on, but without the two blue rods on the end. Then put them together like so in the third picture. Finally connect them to the top railing like shown in the fifth picture.

Step 3: Trigger Assembly

The first two pictures show the "wheel" in which you will insert the rubber band to when firing. The third picture shows the trigger. Hopefully through the next three pictures you can see how the trigger is made. Finally make two pieces like shown in the sixth picture. Next assemble the pieces like shown.

Step 4: Stringing the Rubber Bands.

You then take your rubber bands (in my case I wrap them around each other), then place them in the spots indicated. It is the front of the gun. Then take a rubber band for the trigger and put it in the spots indicated. The rubber bands you use are not set in stone, and you can use any you need to.

Step 5: Making the Arrow. (optional)

For this step, you will need one long gray rod, one red rod, and two brown/orange connectors. After you build the arrow, go outside and sharpen the tip on the concrete(a pencil sharpener might work, but it could break the pencil sharpener). Then for the fletching , grab some paper(I use sticky notes) and cut out your fletching in either a triangle shape, or a sort of half tear shape. Either way works well. You will need four or six pieces of fletching based on what you want, (Like I said before, this is not set in stone and it is very flexible in how you build it) , then tape. It may be hard to see in the seventh picture, but you want to take your tape, wrap it around the rod so that the sticky side faces up. Then, take your fletching and bend a small amount on the bottom side like show in the eighth picture. Make each type two or three times(I used two pieces conjoined pieces, but I made three) . Then take one of each type, and tape them together like shown. Finally take the conjoined pieces, and put them on the sticky side of the rod. Now your arrow is finished!
I hope everyone likes this tutorial, as it is my first. If you find ways to make it more powerful or find errors in my writing, message me. Enjoy, and be safe with it!