Grinding dried chiles in a mortar and pestle?

That sort of grinding only works with seeds and such. Chiles are too fibrous. I use a cuisinart mini mate chopper/grinder for all my spice milling needs. There's a dull side to the blade for grinding stuff like peppercorns, and a sharp side for stuff like chiles. About the only thing my mortar and pestle ever gets used for is grinding kosher salt to a more useful grain size.

You can do this, but it's a fair amount of work.

First, you want to toast the peppers before grinding. This makes them easier to grind, and also deepens the flavor.

Second, chopping them up before starting to grind doesn't hurt.

Third, throw in a handful of coarse salt if there's going to be salt later on in your recipe. It'll help you grind them up.

That being said, I use an electric food grinder these days. 1% of the effort.

If a paste is an acceptable end product (as brought out in comments), then the mortar and pestle will work fine. I believe in toasting most peppers first; straight on the flame, in a dry skillet, or under a broiler all work.

Bring a pot of water with a lid to a boil, salt as for pasta. Prep your peppers by washing and throwing away the stems. Tear the flesh into manageable pieces. Throw the flesh of the peppers into that boiling water, replace the lid and turn off the heat.

Consider how much of the seed you want to keep. More seed will result in a chunkier paste. You might think that the heat is in the seed; it's actually not. Most of the heat of the pepper is in the rib (still connected to the flesh of the dry pepper) and the little connective pieces that hold on to the seed. You can keep that part but strain out the seed by soaking the seeds with the flesh of the pepper. Remove the flesh when it is soft. Pull off any seeds still attached to the flesh and add them to the rest of the seeds still in the water. Now strain the water away using a metal sieve. Now rub the seeds against the sieve. Save the pasty stuff that comes from that, but the (now clean) seeds don't have much else to offer unless you want the texture.

Consider adding thickly sliced cloves of garlic to the water as you bring it to a boil, and mashing them with the peppers. You can do that with slices of ginger too if you want that flavor profile. You can add toasted spices to the pulp as you grind. You can make a very fun paste that way, and you can get creative with it. You can use different varieties of dried pepper at one time, and you can continue to tweak it until you love it.