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Start with roasting a small amount of the origins you think you may want to use in a blend - perhaps a little more for your chosen base(s). Good bases are Brazil, Colombian, Indian and Mexican - anything that contributes to the body and sweetness of the coffee without overbearing the flavour.
Let your roasted beans age appropriately (1-2 days for air roasting, 5-7 days for drum roasting).
Using gram scales or a tablespoon measure (which is about 7g)- you will be aiming to make up blends of about 5 0g total which is enough for 3 double shots.
Choose your base for the blend - start with about 50% (25g). Then add in the other origins you want to put into your blend. 10% is probably the smallest sensible amount you would want to use. Put these in a small bowl or jar and label it carefully.
Now make your coffee normally. If you are wanting to try a number of blends in one sitting, you may want to just taste each blend rather than drink it. Sniff it, slurp it, roll it around inside your mouth. Use your notebook to note what the blend is and how close it is to what you are aiming for. What elements are working for you and what aren't.
If your blend seemed close, try adjusting the ratios - perhaps adding another origin to provide a missing element.
If it was "ok", you might look at substituting one origin for another. If it was nowhere near, look at trying other origins.
You may also want to experiment with different roasts of each coffee in the blend. Roast one coffee a little lighter or darker than the other coffees in the blend and note any differences - maximising the flavour of each origin and creating the most complexity.
Drinking fresh water and eating the odd piece of French bread between tastings helps to clear your palette.