Preheat the oven to 350 degrees F (175 C)
Wipe the chili peppers with a damp paper towel.
Trim the tops off of the chili peppers with scissors.
Cut down one side of the chili pepper with the scissors to be able to spread it open.
Remove the seeds. If you want to make your chili powder less spicy, remove the light colored membranes that the seeds were attached to. In dried chilies the membranes may be stuck to the chili, so you'll need to scrape them off.
Cut the chili into small pieces using the scissors and place on a baking sheet in a single layer along with the cumin seeds and coriander seeds.
Roast the chilies and spices in the oven until a fruity almost chocolate-like aroma comes from the oven (about 1-2 minutes). Be careful not to burn the chiles or or your chili powder will end up bitter. Remove the roasted chili peppers from the oven and let them cool.
Put the chili peppers and spices along with the remaining ingredients in a spice grinder and grind until it forms a fine powder. Depending on the size of your grinder you may need to do this in batches.
Store in an airtight glass bottle until you are ready to use your chili powder.