The first thing that you need to do when trying to draw medieval weapons is to go on and study just about everything that you can about the time. This is going to ensure that you know more than enough about the weapons before hand, and hence when you try to draw them, you won't end up making any mistakes.
At the same time, try and get your hands on the images that historians have created for that era as well. This will aid your imagination and creative process and will help you get on par with exactly what you need and ensure that you end up making the best possible weapons.
Once you have gone on to learn all that you need to about the weapons, the next step is for you to practice some rough sketches. Go ahead and get your hands to open up on paper, and let them create whatever it is that they wish to.
This will ensure that you are ready to draw the weapons and will also make sure that you are not out of practice, when drawing them.
Now go on and try and draw the weapons based on what you have learned, and try and create a weapon based on that. However, if you need help you can look at the pictures in the history books as reference to your art work.