Take one of the two numbers you are trying to multiply and break it down into smaller numbers. For example, if you are trying to multiply 23 by 35, you could break the 35 down into 30 and 5. Breaking the larger numbers down into smaller and more manageable parts, and then adding them back together is the basis for multiplying big numbers in your head.
Now, multiply the 23 by 30. The answer is 690. However, to make this step easier, you could also break the 30 down into 3 and 0. Multiply the 23 by 3 and you will get 69. Now add the 0 to the end to get 690. If you need to break it down even further into one digit numbers, you could multiply the 20 by 3 to get 60 and then multiply the 3 by 3 to get nine. Now add the 60 and 9 to get 69 and then add the zero to the end to get 690.
Now, remember the 690 because you will have to come back and use it again at the end.
Now that you have multiplied 23 by 30, multiply 23 by the 5. 23 times 5 is 115. However, to make this step easier, you could also break it down into 20 times 5 and 3 times 5. 20 times 5 is 100 and 3 times 5 is 15. Add those together to get the 115.
Add the 690 and the 115 together. You can also break the 115 down into 100 and 15. 690 plus 100 is 790. That was easy to do. Now, just add the 15 to it. You will get 805.
Practice this method a few times and you will get faster and faster. Start with smaller numbers, like 8 times 12. You can multiply 8 by 10 to get 80. Then multiply 8 by 2 to get 16. 80 plus 16 is 96. Gradually work you way up to larger and larger numbers. You will be able to amaze yourself and your friends.