The location of this cut will be at the desired height of the leg of your piece. Take the kerf into account, and make a 45-degree cut.
Get as close to your original cut line as possible. The less surface of the top that is cut off, the better. This will allow the wood grain to flow seamlessly down the angle.
You can use a biscuit joiner, domino joiner, or dowel to reinforce the angle. Use wood glue.
Use the off-cuts to create cauls to clamp the pieces together.
And that's it! Just sand off the extra CA glue from the cauls.