First you need to find the files that you want to preserve. Your files might be on physical media devices, like USB sticks, external harddrives, cameras, cell phones, or other storage media.
Download files from online services that you use like your email account and social media sites. (See Resources and Recommendations section for guides on how to do this!)
Your digital files are probably a little cluttered. Now may be a good time to decide what files are actually worth keeping. For example, you could go through your digital photographs to remove duplicates or bad photos. Drafts for documents that you worked on also might be worth tossing if you are satisfied with the final version.
Now that you've curated your collection a little by selecting and decluttering you may want to try to establish some sort of organization system over your files. Establishing a consistent system is very helpful. This will have long term benefits that make your files easier to find and use.
Some techniques you might use include instituting a file naming system. You could also organize files into folders based on the document type and further organize by dates (month or year).
The most important part of this step is creating a system that you actually want to use. It depends on your preferences. Don't try to create something that is too difficult to manage long term.
There's no one solution to ensure proper backups of your data. Therefore, it is a good idea to use a few different solutions to ensure consistency if one solution fails you. This is best illustrated by the "3-2-1" or "LOCKS" philosophies for digital preservation.
Make sure you aren't just relying on 1 copy of your digital files to preserve them. That copy might fail or you may lose access to it. Having multiple copies helps ensure long-term access. Additionally, like the 3-2-1 rule suggests, it's a good idea to store some of those copies in different physical locations to protect against any physical harm to your digital media.
These steps were adapted from Mike Ashenfelder, "The Library of Congress and Personal Digital Archiving" in Personal Archiving : Preserving Our Digital Heritage, ed. Donald T. Hawkins (New Jersey: Information Today, Inc., 2013).