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.
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.
If your paste is thick, add 1 teaspoon of water at a time to thin it out and make it easier to work with.
If you sit on the floor while doing this, it is actually easier as you can let gravity help the pestle pound the ingredients in the mortar.