So is there a nice tutorial which explains how to start mining using FPGA.
Not really, not a "starting from scratch" one.
Or this is so complicated?
Yes, it is quite complicated - in order to work with FPGAs, some additional skills on top of software are required. You need to understand logic design and some (fairly basic) electronics in order to make a successful FPGA.
However, as a learning project, if it's something that interests you, I'd say it's worth a go - you'll learn a lot, but it won't be a quick-and-easy learning experience :)
But start small - first off, write an FPGA which just flashes an LED on and off slowly - that enables you to learn lots of the low-level details without a complex logic design going on. The FPGA equivalent of "Hello World"!
EDIT:
If you've already done some FPGA work, then "all" you need to do is build a SHA implementation, some method of getting data to and from it and a little control logic. I haven't done it myself, but I got about that far along a "thought-design" before deciding I wasn't sufficiently interested to pursue it further.