Summarize the following:
Remember, you have to pay attention to three separate rules when inputting numbers, but this is to your advantage. Very few numbers satisfy all three conditions, and this is where your solution comes from:  Only one of each number in each horizontal row. Only one of each number in each vertical column. The answer of each cage matches the number and mathematical symbol in the upper left corner. Take all of the basic boxes without mathematical symbols first, like "2," or "9" and simply fill in the number. If the box just says two, put in a two. This will take care of the basics and start to reveal other answers. This will depend on the size of the board, but you'll still find them easily with some practice. For example, imagine a 4x4 puzzle. Take a two box cage for "3-." Knowing that you only have 1, 2, 3, and 4 to choose from, you know that there is only one pair that subtracts to three, 4 and 1. While you might not know the order, you do know these are the only two numbers in this row. Make a note of them for later. Every Kenken has some of these "easy" boxes to get you started, such as:  Multiplication/division by grid size and odd numbers usually only has a few answers. Examples include "4x" in a 4x4 grid (1 & 4), "15x" in a 6x6 grid (3 & 5), etc. Two-box addition cages, usually for 3+ or 4+ (can only use 1, 2, and 3). There are a lot of places where two numbers will equally plausible, as you don't know enough about the rest of the board to figure out which number is correct. Write small notes in the upper right corner of the box, allowing you to see which numbers could be in each place, and which numbers have to be there. For example: In the previous step, you noted that you need a 4 and a 1 to make 3-, but you didn't know what order. But if the same row as the top box already contains a 4, then suddenly it's all clear. The 1 goes on top, and the 4 goes below, making your "3-." For example, imagine a vertical row that already contains a 4 and a blank in the top boxes. The bottom two boxes are a cage for "3+," which can only be created with a 1 and a 2. This means the second box must be a 3, even though you don't know what order the 1 and 2 are. The finished cage would look like this:  4 3 1 or 2 1 or 2

Summary:
Always check all three sets of clues before putting in a number. Fill in blank single boxes first. Find and mark the boxes that only have one potential answer. Write smaller numbers in the upper right corner to take notes, crossing them off as they are eliminated. Use groupings of potential numbers to fill in the areas you're unsure of.