Thanks Yevgen, that's pretty much what this topic is for. It's my first large scale build too. I'm tackling every problem which I encounter and try to explain how I solved it or what the solution is. I hope it helps other people to take the step to a large kit/scratchbuild and make their lives easier.
Today I had to go for something else. First of all I glued those beams on the hull so that the hull is narrower aft. That worked, the deck "nicely"' fits in. I'll also add some beams on the stern now, to keep it together and support the deck in that place too. I'll also have to make small cuts for the mine laying devices aft.

As I was a little tired of superstructures, I decided to start one of the main guns. Here are the pics. Instructions were once again pretty scetchy and the part below the turret is not really mentioned or shown. They are resin cast parts, except for the barrel supports. These are styrene sheets. The beam that goes through the barrel, to make it rotate up and down, is a piece of sprue which I sanded to the correct diameter and cut to length. The easy thing about the UHU Allplast is that it just stick styrene, so the resin of the barrel is not stuck to these sheets, this allows it to rotate freely.
The beginning

PE is provided for all the steps etc. But PE is flat and steps and stairs are not... So I made my own from steel rod. Not really a good choice maybe, a true headache to cut and shape, but in the end the result is ok. I'm lucky that Russian ships were also built pretty shabby and nothing is really straight on those ships either. Dry fit of the gun mount. The bottom of the turret was way too large.

The barrel between the two supports. The sprue beam is visible.

Practically I drilled a series of holes in the front of the turret. Then I put inside something that looks like a stitch. After that I wanted to put a brass rod of 0.5mm in front of the stitches and bend them over to become the steps. This method prooved too hard as the glue didn't keep the forces and the "stitch" just came out, it didn't bend as I wanted it.
After that I just started making these stitches, but bent them to correct shape before I mounted them on the gun. Not really perfect either as you'll see...

Here is where I am now. The barrel is just dry fitted until I am finished with all the steps and details around the gun.

In the meanwhile I started another small thing. The Kite Screech guidance radar for the main guns.

Very nice part to make, although a few detail errors are there. Also the resin has a few error in it, so I think I'll remove some parts and replace them by styrene sheets. You can see on of the sides is not so smooth...
That's it for now. I hope the second gun will be better than this one. Maybe I'll think of another method before I start that one. I also have to find a way to fix the additional steps around the gun. It would be strange if I used the flat PE for those as it would be different from all the steps on the front of the turret.