Which slots are proper to install 2x8GB of RAM?

You should populate A1 and B1 first. You can also use the coloring scheme of the DIMM slots:

Notice A1 and B1 are the same color? By installing the RAMs there you will enable the dual-channel architecture.

Due to Intel Lynnfield CPU integrated memory controller limitation, DIMM slots A1 AND B1 must be populated first, or your system may not boot up, since the memory modules in A2 and B2 will not be recognized.

ASUSTeK Computer Inc. -Support- FAQ

Letter commonly refer to channel, numbers commonly refer to order inside channel. In usual case (when manual doesn’t have other explicit instructions) slots must be filled in following order

First slot of first channel (A1 in example)
First slot of second channel (B1)

How to correctly install computer memory

The memory modules are installed into matching banks, which are usually color-coded on the motherboard.

§ Dual-channel architecture