How to install dual channel?

Dual channel slots are color coded. Just pick a pair, or two. Use the same size and speed modules. White id markings will identify the slot numbers, or just check the manual.

A1 + B1

I have one transcend 4gb ddr3 at 1333 MHz,and one Kingston 4gb ddr3 at 1333 MHz...can I put those two to work in dual channel?

Same size and same speed. I would say YES.
Make sure your motherboard supports those brands/DIMMs specifically though, that may or may not be an issue. But, you probably won't have any problems.

I need to ask this, I'm going to install 4 x 2gb on my mb.
Dimm slot colors go orange-green-orange-green, which I find a little strange..
So how to install and and activate dual channel properly?

Board is K9N SLI V2.

Assuming all your RAM is the same size (2GB each, I know) and the same speed (???) and same voltage (???)
You just need to put them all in... and it should be automatic. If not, check your bios for an option and enable it.

Well I'll just shove em all in then and hope for the best.

And yes, they are all the same. Kingston valueram 2gb 800mhz cl5.
But... I'm curious, 3 of them has Elpida chips and 1 has Kingston, should I be worried?  

I don't think soo...... give it a try... then run some tests like memtest86+ and figure it out...

It is dependent on the motherboard, AS WELL AS, the windows bit you are using. I have an AsRock Pro3-M Motherboard, after reading through the manual properly, I found out that windows 32-bit limits the ram to under 4 gigs. But 64-bit has no limitations. Maybe it's the windows your using, try upgrading to windows 64-bit.