When making a pattern there are several methods you can use. The first is the most traditional and for me the most tricky - using exact measurements to make the pattern. If you are good at modern dressmaking and able to follow a real pattern then I would suggest that you try using "The Medieval Tailors Assistant", as the making suggestions in this guide are aimed at those with only basic sewing skills.
You will need large sized paper – I normally use left over wall paper, or wrapping/packing paper, or news paper sellotaped together; but wall paper is my favourite. You will literally draw out the body measurements until they fit together. Then you add a seam allowance and moving around room to get to the shape of the pattern you're aiming for. All I can say is try your best – if you're not sure of the pattern use old/spare/cheap material to practice on first – or make the pattern even bigger – you can take it off but you can never put it back on! Patterns, such as hoes, work best when you just wrap material around the leg and cut the pattern from that fitted pattern. Remember to always keep a record of your changes on your paper pattern for future reference.
The second method I prefer is to take the pattern from someone else's made up garment and alter the pattern to fit the new owner. To do this; you lay the garment on the floor, look at the shapes of the panel and reproduce them on the pattern paper, then decrease or increase the pattern size as necessary, remembering to add a seam allowance at the end.
Less commonly, I can often just look at a person and just draw out a pattern without measurements, when the garment is a doublet, coat, or something of that nature. This does seem to most others a very unusual technique and not one I would recommend unless you are willing to make mistakes.
After you have your pattern, decide on a seam allowance, this will depend on your own method of sewing and preference. Anywhere from 1cm to 2.5cm will be more than adequate. If you're not sure, try a larger allowance and work your way down. Mark on all of your patterns, with a dashed line, the seam allowance you have made for future reference, as well as the article and piece information.