Guidelines are evidence-based, and often are informed by systematic reviews or meta-analyses (the highest level of evidence on the evidence-based practice pyramid).
With significant clinical data, the guidelines advise how to care for patients with certain conditions based on what has been most effective for that condition / population.
Guidelines offer recommendations, but you must pay attention to patient-specific information that may not be addressed, such as co-morbidities, patient preferences, and an ability to pay for the recommended treatment/procedure. You also need to make sure you are using the most current guideline available as guidelines are updated to reflect best practices as new treatments or procedures are developed or as the evidence-base changes with new data. You may also need to check multiple guidelines since some conditions have multiple ones available (e.g., Diabetes, dyslipidemia).
As you're reading guidelines, you will often notice that the recommendations are given a letter or numeric grade. These indicate the level or quality of evidence. Below are links to some of the common guideline grading systems (please note this is not an exhaustive list). It is important to look at the specific rating system the guideline uses and how they may have adapted the grading system they used, especially since many organizations have developed their own rating system for their recommendations