Everything I'm using. Note you don't have to use exactly the same stuff I use. This is just what I could lay my hands on, which means it isn't the necessarily the best items to use.
Tape 2 pencils together. Tape: 1 in. from the eraser, 1 centimeter from the unsharpened end of the pencil and in the middle.
Repeat the step until you have a pair of a pair of 2 taped pencils
Tape the two pencils together so it looks like a lowercase "T" or a cross. One pair of pencils would be taped to the middle, about a centimeter down from the unsharpened end of the 2nd pair
Take your pen apart. Ball point pens are easy to take apart. Use your nails or even teeth. You can use skinny items to shimmy it out to pull the back out.
Be careful when you're taking the back out of the pen because you'll need it later, so try not to use your teeth unless you have no other way to get it out.
Discard the spring of the pen if it's a click-type pen. There are ways to use it though, like faster or further shot enhancer. I won't be using it in my model.
You may want to use the unsharpened part to be your front. When you shoot the ink cartridge or other projectiles the metal rim that is attached to the eraser can disturb or offset your shot.
Stretch your rubber band between the gap of your horizontal pair of rubber bands. So it goes between the two erasers and the two unsharpened ends.
It may be better to use the thicker rubber bands for this part because you don't want it sliding, but a smaller rubber band would help give your drawstring better power.
Wrap tape around the rubber band in the center to form a sling or a pouch for the projectile. Don't tape the rubber band to the body of the crossbow or you don't be able to draw it back
If you want you can take a square of cardboard or fold over a piece of paper a few times and tape it around the rubber band to improve cradle.
Tape the tube of your pen and add tape it to the body of the crossbow, have the drawstring rest on top.
Wrap a rubber band under the barrel of the tube. You can also thickly wrap tape to go with it. It raises the barrel so it shoot higher and better.
Take your ink cartridge or what ever projectile you decide to use and slide it in the end of the barrel that doesn't face you. The point will be facing out. Depending on what you use it may stick out.
Other projectiles that may work: toothpicks taped in a javelin type way, tinker toys, or even sharpened sticks.
Some projectiles can be loaded from the front of the crossbow, it may be faster. Experiment with different types of projectiles.
Pull the drawstring back until the projectile is resting in the pouch of the cradle. Make sure you can slide it back fast. Also make sure you can slide it in nice and easy if you decide not to shoot.
Take precaution! If you decide to use and ink cartridge from a pen, and there is still ink inside, you may draw or leave marks on whatever you hit!
To finnaly shoot, first check to make sure your projectile is snug inside the barrel, if it's too loose it won't shoot straight. If it's too large for the barrel it will get stuck.
Pull your projectile back with your index finger under the rubber band and projectile. Have your thumb behind it for balance. If released correctly it will shoot fast and far.
Be careful! The crossbow should never be taken to public places or used for the specific purpose of hurting people you don't like. It can and will damage something that you have shot.
Practice your shot and experiment with different projectiles. Also try changing it and adding different modifications to the actual crossbow.
My finished Office Crossbow
Big thanks to Wikihow for the inspiration and basic help with the steps.