So, you've seen an amazing YouTube video of someone's Christmas lights flashing to a musical soundtrack and are thinking, "I could do that! How hard could it be?" Are you sure you want to know the answer?
It's actually not that hard at all if you have lots of disposable income, are an electrician, write code, and are willing to sacrifice months of your free time. OK, that's a lie: it's still hard under those conditions, but at least it's possible. Seriously, this project could be a year in the making.
Degrees of DIY
What? You still want to know more? Your enthusiasm can't be dampened so easily? Well, here you go then: as with many other DIY projects, this one exists on a continuum between cost and level of difficulty, that is, the more you pay, the less you have to DIY, and vice versa.
If you don't know much about electronics, can't solder, etc., you might want to spring for the expensive-but-simpler option, which consists of a software package for your computer and all the requisite hardware. You just choose which lights to hang where and when to have them flash, fade, shimmer, and so on, and then program that information into the software in .1-second intervals.
It'll still take a good bit of time. It says something that one of the companies that manufactures the control boards necessary to coordinate these light displays also offers classes on how to use them.
Going All In
If you do know a thing or two about either computer programming or electrical engineering, you can DIY those elements to whatever extent you're able. If you go that route, you officially have a new hobby. Allow it the investment of time, money, and devotion it deserves. The first time you do this, you will likely have setbacks. Many of the little bits of knowledge you need to bring such an ambitious project to fruition will be acquired through trial and error.
Luckily for you, others have blazed a trail. Sites such as doityourselfchristmas.com should connect you with people who share your passion for creative expressions of holiday spirit and are more than willing to share what they've learned over the years.