The vast majority of espresso is brewed from a blend of coffees from different origins. Single-origin espresso (or SOS - single origin shot) is gaining popularity, but I feel it is more of an intellectual culinary pursuit and not best practice for an ideal espresso. So how does one create an espresso blend? First you need an idea for what you want the blend to be. Do you want it dark roasted with a carbony bite, or medium roasted with more of a caramel-coffee flavor? How thick of a mouthfeel? how acidic? how much caffeine? what flavors? fruity? spicy? floral? chocolaty? nutty? bakers chocolate with blueberry and jasmine? Once the ideal is formed, start with low-acid South or Central American coffees for a non-overbearing base with good body and sweetness.
The coffees you select next will be a combination of coffees that bring you as close to your ideal as possible, but be wary because most coffees will contribute something positive and negative to the blend, the blender needs to minimize the negatives and maximize the positives. Generally speaking one can use some Central American coffees to increase acidity and add specific flavors, use some African coffees for complexity, brightness and to add specific flavors, use some Indonesian coffees to increase richness and body and to add specific flavors, and use Robusta coffees to increase crema stability and caffeine content. Next, Cup (taste-test) possible coffees to decide on which specific origins to use. Then mix those origins together at different ratios to decide of what proportion of each coffee to use in the blend. The coffees should be roasted individually to their peak and then mixed together in the determined proportions (not blended then roasted). Finally, make some espresso with your new blend, made any needed minor adjustments needed, make some espresso with your new blend, and enjoy.