• Decide what size of cannon you want to cast (light, field, heavy or even bigger)
  
• Shape its barrel (mortar, howitzer, long-barrel)
  
• Choose decorations and inscriptions
  
• Decide what material - (bronze, brass, iron?)
  
• Cast off the cannon, keeping an eye on the melt and temperature (make no mistake because the casting will come out brittle)
  
• Choose the caliber of the barrel and drill it (the thinner the walls, the larger the ball but also higher the risk of explosion of the gun)
  
• Polish the finished barrel
  
• Build a gun carriage for it (for ships, field, fortress, siege)
  
• Test your cannon on fortresses, buildings and ships.
  
• Kings and generals will want to buy cannons if they are good.

You set up a cannon foundry. You have to go through the entire process of creating an old cannon from the very beginning. Build your workshop and start production. You start with the most primitive medieval guns. But you're still developing. You improve the workshop and discover new technologies. You build both light, field and heavy guns. After selecting the size of the gun, you set the shape of the barrel and its length. You also decide whether it will be a cannon or a mortar. Once you have made the cannon mold, you go into a risky casting process. If the casting succeeds, you can drill the barrel (the best guns had the barrel drilled only after casting!). Be careful not to overdo it with the size of the barrel in relation to the thickness of the barrel walls! Later you decide on which carriage to mount the gun. Heavy guns require a strong carriage, so their mobility will be reduced. Then test your gun - is it accurate? how is the range? does the cannonball fly flat or steep? how much gunpowder can you load (velocity of the cannonball)? how effective is it in destroying various targets?
This all affects the price your guns will get on the arms market.