Remove the seeds and soak the guajillo chilies in water for about 20 minutes to allow them to soften.
Remove the tough outer portion of lemongrass and mince the bottom tender white part (remove the tough top portion).
Remove the stems from the Thai chilies and chop. Remove the seeds for less spice.
Chop all ingredients as small as you can get them to make it easier to crush into a paste.
Combine all ingredients in a mortar and mash together using a pestle until a paste forms.
If the paste is too thick, add a teaspoon of water at a time to thin it out.
Store in the fridge for up to 3 months, or freeze in 1 tablespoon cubes for up to one year (keep in a freezer safe bag).
Notes
Using a mortar and pestle provide the best results as it lets the oils come out of the ingredients - however, you can use a food processor to make this quicker and easier.
Add more or less Thai chilies depending on the heat level desired (Thai chilies are very spicy).
If your paste is thick, add 1 teaspoon of water at a time to thin it out and make it easier to work with.
Replace the fresh lemongrass with lemongrass paste if desired.
Toast the whole coriander seeds for a slightly smokier flavor.
If you can't find guajillo chilies, dried ancho chilies can be substituted.