Just keep this site open in your browser and it will use 100% of your CPU by doing SHA256 hashing. For multi-core CPUs open this site in multiple tabs, one tab per core. Let your CPU heat up for at least a couple of hours to ensure your thermal compound has bonded well with your heatsink. Keep a close eye on temps to ensure they don't get too high. If they get too high you may need to do a better job of applying your thermal compound or get a better heatsink. The longer you can run it without problems the more stable your system will be. Some people recommend up to 2 days for the full burn in process but this time will vary based on what type of thermal grease you are using. When you are done simply close this page.
We are not responsible for damage caused by this burn in process. It is up to you to make sure you do not overheat your CPU.