The obvious response to that question is, "A whole lotta work!" To quantify that, it usually takes about 18 months with two or three people working almost full-time in order to submit a charter school application and see it through to approval.
There are different stages as noted here in a handy resource called Start a Colorado Charter. The really busy times are right before the application is submitted and then during the hearing process. Throughout the entire process it's important to build momentum by marketing the school in the targeted community.
Marketing will consist of getting the word out through fliers, information tables at local businesses or community events and door-to-door. The applicant should decide early in the process what characteristics the ideal student will possess. If the school will be targeting students whose first language is not English, the fliers about the proposed charter school should be translated.
The bulk of the preparation time should be spent learning, both by reading and speaking with people. A lot of what's important to know about opening a new charter school isn't available in written form. The knowledge is gained through exposure to information at workshops and personal conversations.
There are numerous resources online through the CO Dept of Education and the League of Charter Schools' websites.