The 5:1 Ratio
Muscle growth is a much slower process than fat loss. This is especially the case for hardgainers. Consequently, the ratio of time spent in a surplus (gaining mass) compared to being in a deficit (cutting) should reflect this.
For average guys, a 4:1 ratio is a pretty good guideline. For hardgainers, I prefer a 5:1 ratio. So, if you bulk for 15 weeks that equates to a 3-week mini cut.
If you struggle to build muscle, you must allow yourself sufficient time to add a significant amount of mass. This means you must bias your efforts in favor of muscle-building phases. This allows more time for growth. This is the hardest part of the equation for tall, skinny guys, so it deserves the majority of your attention.
You Still Need to Diet Sometimes
As a skinny guy, you might think that you'll never need to cut. Sorry to spoil the party, but there will eventually come a time when it'll be a necessity if you've been eating the quantities of food needed to grow.
Sooner or later you'll get a little fluffy and your abs will disappear. At this point, you're not in such a good position to gain muscle. Chances are, most of the added weight will be fat at this stage. So, take a mini-cut. Lose fat. Improve your body's capacity to gain pure muscle and then start over with another bulk. This will allow you to build more muscle (instead of fat) in the long-term.
Progress Won't Be Linear
You have to be mature enough to accept this. Push for as long as you can and know when it's time to switch gears before pushing again. The process flows through these cycles, but the trend should always be upwards. Use the 5:1 ratio to guide you in planning your training and nutrition to build your best body.
For a naturally skinny guy, this looks like 40-45 weeks of the year building muscle and 7-12 weeks of cutting.
Obviously, there will be exceptions to this rule. Once you've built a substantial amount of muscle, you might be more inclined to show off your physique and want to be shredded for longer periods of the year. This requires more time spent dieting.
However, obsessing over seeing your abs year-round as a relatively small guy is pointless. If that description is an accurate reflection of your current situation, trying to stay super lean is a terrible idea. It'll sabotage any chances you have of gaining a lot of muscle. In fact, it's a one-way ticket to staying small and weak.
Commit to putting some size on. Build a base and then cut down to reveal your new and improved body.
The One Thing You Don't Want To Do
Please don't misinterpret this as an excuse you to get fat. There's only one thing worse for a hardgainer than being skinny: being skinny-fat.
This 5:1 ratio is designed to maximize your chances of building muscle, yet it still accounts for the need to occasionally diet away any fat gained while bulking. The good news is, skinny guys tend to be able to drop fat rapidly. This is why you can get away with the 5:1 ratio and regular guys are better off with a 4:1 (or lower) ratio.
The typical hardgainer's metabolism means that in only a few weeks of aggressive dieting they can strip away all the fat accumulated in mass-gain phases. Watch what you eat for a few weeks and you'll drop fat. You probably don't even need to do cardio.
See? Being a hardgainer isn't so bad after all.