Plan, plan, plan. I cannot emphasize the importance of this step enough. Much like what Benjamin Franklin said, “if you fail to plan, you are planning to fail” — a step I have found many people skip when building a home recording studio.
During this phase, you can reverse engineer your studio for the best overall result within the confines of the budget you have available. Start by figuring out the desired end goal. For example, if mixing music is the goal, the environment created should focus on listening. If recording is the end goal, your plan should target getting equipment that helps eliminate outside sounds like the air conditioner.
A recommended portion of the planning phase is to determine a budget that works for you. To create music competitive with major record labels that have commercial studios readily available, you should aim for quality equipment without entering into the overkill territory. On average, I would say that those looking to build an in-home studio should expect to budget around the equipment you have available already and the ultimate end goal of the studio. Reading reviews can help you find quality equipment within your budget.