FIND - Guajillo chilies are fairly commonly found at most well-stocked Mexican or Latin grocery store. In areas catering to large Mexican populations, they may be found in well-stocked big-box grocery stores as well, though this is a rather hit-and-miss (chipotles and anchos tend to be much more common).  While guajillos are an important Mexican ingredient, they are not a particularly common in the “Mexican” section found in many grocery stores, which tends to lean towards pre-packaged Tex-Mex.

The popularity of Mexican cooking has grown a great deal in recent years, and as a result, many Mexican dried goods are quite easy to find online.  There are lots of small scale chili growers selling guajillos and other chilies online, both through big e-commerce sites like Amazon and through smaller independent channels.  Look for retailers that emphasize freshness and quality, and are able to ship to your area without too much difficulty.

Hardcore DIY fans with access to fresh guajillo mirasol chilies can make dried guajillos at home (it helps to have good knowledge of dehydrating food). That being said, outside of Mexico, fresh guajillo chilies tend to be far harder to find than dried ones anyway. It’s also worth noting that mirasol chilies are a broad group of chilies with many different types and flavours, and the guajillo is only one of many cultivars within this group.

CHOOSE - Guajillo chili peppers should be dry but supple, with a surprisingly flexible and soft skin (you should be able to bend a guajillo chili somewhat without breaking it). Extremely brittle and damaged peppers should be avoided if possible, as they will have lost a great deal of flavour. Likewise, overly soft or damp peppers may have been exposed to moisture and should be avoided. Guajillo chilies tend to be a lot more durable than many dried peppers, but in the event that you do find broken or cracked chilies they can still be used as long as they’re still soft and flexible. The colour should be a vibrant medium-red to reddish-brown colour, often with brighter red patches. Very small pale yellowish or orange patches sometimes form on the surface during the growing or drying processes, especially near the tip. These do not affect the flavour of the pepper. Thin, dusty looking checks, marks, or lines are often present on the surface of the peppers – these tiny dry spots form on the peppers (you can often see them on fresh jalapeños) as they grow and do not affect the quality or the flavour.

Guajillo peppers tend to be quite long; around 5 inches (13 cm), though they can vary somewhat in size. Variation in size does not particularly affect the flavour, quality, or heat of the peppers, though you may want to choose a consistent size if you plan to use recipes calling for a certain number (rather than weight) of peppers in order to achieve consistent results.

Crushed or ground guajillos can be quite difficult to find with accurate labeling, but they can be made easily at home by stemming and pulverizing whole dried chilies (with or without seeds, depending on how you want to use them). Note that the dried chilies benefit from additional toasting and drying before grinding, as they tend to be quite soft and pliant in their dried state.

PREP - Guajillos require a low-to-moderate amount of preparation, though the particular method(s) necessary will vary from recipe to recipe. As a general rule, the stems are removed and discarded. The pepper may be cut or cracked open to discard the seeds, though this is often optional depending on the desired heat of the final dish (the seeds and membranes are the spiciest part of the pepper).  Don't feel that you need to get every little fleck of membrane out of the chilies - you'll go nuts trying to get it all out, and you really don't need to worry about it that much.  Any seeds that get missed are generally easy to pick out once you rehydrate the peppers too (see below).

Toasting: Guajillos, likes many other dried chilies, are often toasted in a dry pan to accentuate their flavour. This is generally done in a hot skillet, and takes only a couple of minutes.  The chilies are ready when they're supple and very fragrant - but take care not to scorch them.  A non-traditional but very effective alternative involves microwaving the chilies for about 20-30 seconds.  As with the skillet method, they should end up supple and very fragrant.  This method is very easy, but does tend to leave your microwave smelling strongly of chilies for a while.

Rehydrating: Guajillos are are also commonly rehydrated in warm water or stock, then used (plus or minus the flavourful soaking liquid) in a recipe. The particular method necessary are usually specified in a given recipe, but these basic principles can easily be used by the cook looking to create or adapt a recipe featuring guajillos.

Grinding: Whole guajillos chilies can be pulverized in a spice grinder, food processor, or large mortar and pestle to create chili flakes, but the flexible and leathery nature of the chili skin makes this a little more difficult than for other peppers. Consider carefully toasting your chilies in a low oven before grinding them to make the process a little easier. Discard the stems before crushing, and (if desired) remove the seeds for a milder end product.

STORE - For best freshness, keep chilies in airtight containers in a dark pantry.  Light will degrade the quality of your chilies over time, so if you have to store them out in the open, do so in an opaque container.  Stored product pests like dermestid beetles love chilies, so trust me when I say that you want your packaging to be genuinely air-tight, or they'll find their way in through small opening and ruin your peppers.

Substituting for chilies is always a bit complicated, but there are a few basic options available.  In my opinion, cascabel chilies are the best single chili substitute.  They're fairly similar in terms of heat and flavour profile, though they tend to be a bit more fruity.  Puya chilies are similar as well, though sharper in flavour and generally quite a bit hotter.  A good way to emulate some of the earthy, smoky fruitiness of of guajillos is to combine cascabels with pasilla negro chilies.  A 2:1 ratio of cascabels and pasillas is a good place to start, though you may want to tweak the ratio depending on what flavours you want to emphasize.

I have seen ancho chilies listed as a good substitute, but I personally disagree with this.  Anchos are easy to find, similar in spiciness, and they have a pleasant fruity/smoky flavour, but they lack the sharp, somewhat acidic character of the red mirasol chilies, instead favouring a chocolate, smoke, and leather flavour profile.  Substituting anchos for guajillos tends to change the character of a recipe pretty significantly.  That being said, a little bit of ancho mixed with cascabels (similar to the pasilla/cascabel combination) would probably make a nice combination, and a decent substitute for guajillos.