Number the sections by the decimal point numbering system:
1.0 Title of first main section (usually Introduction)
1.1 First subheading
1.2 Second subheading
2.0 Title of second main section
2.1 First subheading
2.2 Second subheading
2.2.1 First division in the second subheading
2.2.2 Second division in the second subheading