I am thinking about buying an ATtiny84A, which fits perfectly into my project. I want to embed it into a watch that I am designing right now. But I want to buy an SMT version so I have less space to consume. What do I need for programming it through USB? Special Atmel chip? Or just 2 digital pins from USB directly into it? Thank you very much. :)
You need to design your device to permit In Circuit Serial Programming - ISP. This will let you continue to refine the program once the chip is in the circuit, and it will also be your primary method of programming and testing small production quantities.
Basically this means that the ISP and reset pins of your processor need to be able to be externally contacted - either by a header, pads for "pogo pin" spring contacts, an SOIC test clip, or perhaps micrograbbers hooked onto individual pins.
It also means that anything connected to those pins needs to not interfere with or be harmed by the programming process. That particularly means that anything which could drive any of these pin needs to not do so while the MCU is being programmed, nor load them, nor get lastingly confused by operations the programmer does on them.
You'll also need an ISP programmer. Proper ones are sold, but beware of cheap clones and marginal designs as these often give users plenty of trouble. Fortunately, you can use an everyday Arduino running the ISP sketch to program ATtiny chips. Do beware though, that if your target is powered at 3.3v it is not safe to use a programmer with 5v signals on it - you might get away with it a few times, but eventually it will cause issues. So if you want to use an Arduino to ISP a lower voltage target, you either need an Arduino that runs at 3.3v, or to implement level shifting.
Do spend some effort to verify that you have the software support resources you may need for the ATtiny84A - it's one of the less common chips so it won't have the same degree of community support.
A further possibility is to use ISP to put a serial (or other) bootloader into the chip and then rely on that, however this won't cover all eventualities - if you discover you need to change the fuses, you'll need ISP for that, as it's something a bootloader can't do.
You need an Surface Mount Adapter Kits
This is a soldered version for breadboard prototyping.
There is a model that doesn't requires soldering:
This cost US$70 (http://www.logicalsys.com/painfo-vpasp-vb.asp?adapter=pa8tss-ot)
After that, you mount your SMD on a breadboard and program the ATtiny as normal.
Of course, it doesn't work if your ATtiny is already soldered in the watch.