NZ Maths Home Page - Curriculum Elaborations, under this 'Key Ideas of Mathematics'. These are no finite, they are infinite. Big idea is CENTRAL to the learning of mathematics, big ideas make connections. Big ideas develop a deep understanding of mathematics.
You need to know the curriculum. Do you research google: 'Key mathematical ideas', you must understand these ideas. Books such as Van de Walle are great.
Step 2: Think about your students
Think about what they bring to the classroom culturally and mathematically.
Put current knowledge and interests at the centre of your instructional decision making
How can you best present mathematical concepts can match your students prior knowledge base?
Step 3: Decide on a task/ problem