Use scissors to cut the stems off the guajillo or ancho chiles, then cut each chile in half and remove the seeds. Toast the chiles in a dry skillet over low heat with the cumin, coriander, and fennel seeds until fragrant, about 30 seconds.

Tear the chiles into small pieces and transfer them to a spice grinder. Add the cumin seeds, coriander seeds, fennel seeds, garlic, paprika, oregano, allspice, salt, and sugar. Grind into a powder, working in batches if necessary.