The size you make the cones is up to you, but you don't want it too big or it will be hard to handle. It also depends on the size of the freezer bags you start with. I usually cut the bags into 6" squares. Use a straight edge to make sure your corners are 90 degrees, as this will help in the actual folding of the cone.
Step Two: Make a funnel shape out of the square

After you get the funnel exactly how you want it, you will need to tape along the side edges where the plastic overlaps. Be careful not to move the plastic when you do this, or you will have to start over again. It's good if you already have a few small pieces of tape ready instead of trying to get it from the dispenser at this time. Tape all the way from the bottom to the top of the funnel.
Step Four: Filling the cone
Now you will want to put your prepared henna paste in the cone. Using a small spoon* seems to be the best way to do this. Carefully place a spoonful of henna paste into the cone, and press your finger lightly against it (from the outside) as you remove the spoon. Repeat until the cone is about half full. Filling too much can cause the cone to be hard to handle and burst due to excessive squeezing. *Note: Never use anything metal with your henna paste - use plastic or glass.
Step Five: Closing the cone

And that's it! It takes a little practice, but making your own henna paste and your own cones is less expensive, and creates beautiful henna art without a lot of work.