I carry/make a few blends... it can be very 'easy' or difficult depending on the coffees you have and goals in mind. The most difficult to 'perfect' was a espresso blend... I found the espresso extraction process brought out different flavor profiles that were more difficult to balance. I first start with a goal... what am I trying to create this blend? For example, when I created my 'breakfast' blend called Harmony my goal was to select a balanced coffee with a touch of acidity to wake up the taste buds. My first decision was to select the base coffee so I roasted all of my coffees and brewed a pot of each of the coffees. I decided in this instance my Colombian was the right base - it had a nice balance, a touch of sweetness and a bit acidic. The key was I was looking for a base with acidity - but did not assault the taste buds with super heavy duty acidity. So, I mix started with a 50/50 mix of Colombian/another coffee... Colombian/Brazil, Colombian/Guat, Colombian/Sumatra... etc. Once I find the 'second' coffee that complimented the base - I play with ratios... 25/75, 50/50, 75/25... Sometimes I find two beans is sufficient, often it may be missing sweetness or body, etc. I try and get the first two beans as balanced as possible.. for example say I land on a 50/50 mix. I will brew a pot of 50/50 mix and slowly add the third coffee... 50/50 mix and then say 25% of the mix volume with 3rd coffee. It's literally a ton of trial and error... I bet I make 30-40 blends on average before I am happy with the results. Once I decide on the final mix - I make the decision of I can pre/post roast blend the coffees. I take the ratio I landed on, blend my green coffee to that ratio and roast. Then I compare the two pre/post roast blended cups and decide which is the best - only then does it go into production.
I never use 'filler' beans or beans that I wouldn't sell as a solid single origin drinker. I figure just like any good chef, start with superior products and you end up with a superior product.
That's my 2 cents... have fun - its coffee!
so detailed post