I am no coffee expert but I assure the following will help you create an excellent and consistent roasted coffee beans.
Pick coffee according to civet cat preference. Those civet cats are extreme picky eaters. Ingesting only coffees at peak of their ripeness. A simple but hard to follow rule – pick only ripe coffees and leave all the green behind. Farmers want an easier one time harvest, gathering all cherry, greens and reds.
Perhaps getting pure red ripe coffee cherries processed properly into green beans is impossible. You can either, purchase the sought red ripe cherries, manage the harvesting on your own or with your most trusted personnel or pay a premium for it. There is no way to tell whether the green beans all came from red cherries or a mean to separate the mixture.
Hire a number of expert coffee sorters. Manual sorters I mean. They must know which beans are bad and which are good and must able to do their job patiently. Sorting beans the whole day is a very boring job.
Never trust even the highest graded beans. Passed them under the sorter’s meticulous eye. Remember, a single black bean might be enough to make a cup bad, a whole batch bad and drive away 80% of your customers.
Check the bean moisture content before roasting. Too much dried coffee beans roast faster and might get burnt before the specified time. Too moist roast slower, perhaps won’t get enough flavor development.
Dry it further if too moist or let absorb moisture if too dry. You need a sophisticated equipment to do rapid moisture checking. It is expensive but worth the investment.
Blend after roasting. Roast varieties separately, Robusta, Arabica and Liberica. They have different sizes and densities. Smaller bean roast faster than larger. Imagine the outcome if they are roasted at the same time. A mixture of burnt and properly sorted beans or properly roasted and under-roasted
If the blend involves a single variety coming from different areas. Separate them and roast separately. Each sure have different flavor profile. Explore the flavor potential of each bean and do the roasting and blending process.
Train a group of coffee tasters to do the blending. You yourself can do it. However, taste is subjective and affected by mood, eating habits, health etc… A team of coffee tasters can do a more objective and repeatable evaluation. Two heads are better than one, plenty are better than just two.