1. First, make clear the requirements for the apartment type, height and bearing capacity of the self-built house. According to the requirements and local building codes, select the appropriate structure type, such as reinforced concrete structure, and determine the thickness and reinforcement of 3m floor to ensure that it can bear the expected load.

2. Secondly, for 3m floor slab and 6m main girder, the section size and reinforcement are calculated to ensure the stability and bearing capacity, and the connection mode between floor slab and main girder is designed to ensure the overall stability of the structure. Considering the performance requirements of fire prevention, waterproof and sound insulation of floor slab and beam, the corresponding design is carried out.

3. Finally, draw the design scheme into detailed construction drawings, including plan, elevation and profile, and invite professionals or institutions to review the design scheme, adjust and optimize it according to feedback, carry out construction according to drawings and specifications, and conduct acceptance after completion to ensure the quality meets the requirements.