Seems like there's a lot of back and forth here, and I too was confused by this for a long time, so let's go through it all at once. Mods, feel free to let me know if I'm breaking the rules by, you know, posting the rules...
To summarize for those who just want the low-down:
With wireless disabled, the following gets you +2 Accuracy only:
1. Smartgun system, either externally fitted to top or bottom slot of ranged weapon or internal
2. Smartlink (not to be confused with image link), installed/implanted in:
2a. Imaging device, OR cybereyes/natural eyes with datajack
With wireless enabled, and the smartlink installed in an imaging device (NOT installed in cybereyes or impIanted in natural eyes), get +1 dice pool bonus.
With wireless enabled, and the smartlink installed in a cybereyes or impIanted in natural eyes, get +2 dice pool bonus.
For those of you who want to reference the rules for the above, read on.
Requirements:
p. 433, Firearm Accessories:
"Smartgun System: This connects a firearm or a projectile weapon directly to the user’s smartlink."
"The smartgun features are accessed either by universal access port cable to an imaging device (like glasses, goggles, or a datajack for someone with cybereyes) or by a wireless connection working in concert with direct neural interface."
"Retrofitting a firearm with an internal smartgun system doubles the weapon’s price and adds 2 to its Availability. An external smartgun system can be attached to the top mount or underbarrel mount with an Armorer + Logic (4, 1 hour) Extended Test."
Note that the external smartgun system only costs 200nuyen, more often than not much less than the 2x weapon cost. If you have a free slot (and you should, since it comes with a camera with capacity 1 for vision enhancements), this will often be a more cost effective solution if your gun of choice does not already have a smartgun system.
All right, so you need a smartgun system, a smartlink, and an imaging device. Now that we know the basics of how a Smartgun system works (the first of the three requirements), let's look at the smartlink.
Page 444, Vision Enhancements:
"Smartlink: This accessory works with a smartgun system to give you the full benefit of the system. The smartgun will tell you the range to various targets, as well as ammunition level (and type), heat buildup, mechanical
stress and so on. Without a smartlink, a smartgun system just sends out data that isn’t received by anyone and has no effect. A smartlink installed in a natural eye or in a pair of cybereyes is more effective than a smartlink installed in an external device; see Smartgun System, p. 433."
Page 453, Eyeware:
"Smartlink: An implanted version of the smartlink vision enhancement (p. 444). Note that a smartlink installed in a natural eye or in a pair of cybereyes is more effective than a smartlink installed in an external device: see Smartgun System, p. 444."
Just noticed that this needs to be changed in future printings; the last sentence should read "Smartgun System, p. 433".
All right, fairly straight forward so far; you need the smartgun system on the gun, and a smartlink in either gear or augmentation form. Finally, let's take a look at gear and DNI;
Page 443, Optical and Imaging Devices:
"From wiz shades to bulky goggles, basic vision enhancers are available in several forms. All such devices have—and by default use—wireless capability, although you can use a universal data cable for most of them. Each device has a Capacity that allows you to add vision enhancements. Some of these devices are worn, while others are external sensor devices or even attached to weapons."
All right, still pretty straight forward; get any of these devices, put a smartlink in it, and you've got what you need for the basic features of the smartgun system. To get the full benefits of the system, you need to have an implanted smartlink, which can be done in one of two ways:
Page 222, Direct Neural Interface
A direct neural interface, or DNI, connects your brain to electronic devices. Direct neural interface (along with a sim module) is required for VR use, but DNI is also useful for AR in that you don’t need any additional gear like earbuds or an image link to see or hear augmented reality objects. How do you sign up? You get DNI by wearing trodes, or having an implanted datajack, commlink, or cyberdeck. And if you’re using a sim module along with your DNI, you can get a sort of “super AR mode” where you can also feel, smell, and taste your AR experience.
Notes:
Pretty much every character example in the book has both smartlink and image link systems on their imaging devices; this is not technically required by the RAW, but it could be argued that the smartgun and smartlink by themselves are not enough to "display" the data provided by the system. I am personally of the belief that it is perfectly acceptable to install a smartlink IN the camera provided by the smartgun system, and connect the two together through a universal data connector. You would only get the +1 bonus if looking through the scope, though, at least from my point of view (hah!).