As a beginner myself may I offer a couple of hints I've picked up over the construction of my layout. It is a German HO layout, small terminus to fiddle yard, some 16 feet long plus fiddle yard.
I wanted to build the layout as well as I could so decided to ballast it once the track was laid and working. My word, I had no idea how time-consuming it would be to make a million tiny stones go where I wanted them to go and nowhere else . My first tip would be to watch some of the excellent Youtube clips showing how those who have done it before do it.
I used the plastic ballast layer which worked very well although it applied more ballast than I would have liked. My problems arose when I started applying the PVC glue/water. I rushed it which led to several problems I could have avoided. I used a large syringe and the first thing that happened was that it washed away the ballast on either side of the track. The ballast between the rails was contained by the track and was OK. Has I looked at the clips first I would have seen the advantage of spraying the track with water to soak it first to help the glue/water mix to flow although I did remember to add some washing up liquid.
Having soaked, as I thought, the ballast with glue, I found gaps all over the place where the ballast had been washed away or patches of soaked ballast had spread further out, very unprototypical!!
My next mistake was to try and repair the damage before the glue had fully hardened leaving several more gaps!! 24 hrs, even 48 hrs would be my suggestion.
I then spent hours and hours scraping glue hardened ballast from the places it shouldn't have been and filling in the many gaps where the cork underlay was showing through. It finally looks OK although not as good as I would have wished.
Then there are the points!! Just see how the experts do it, is all I can say, it takes a lot of time and knowledge to get it right.
My second tip, I'm almost embarrassed to mention it, is Read The Bloomin' Manual or, in this case, the advertisement properly. I am the not so proud owner of two excellent models of warehouses ....... in 'O' gauge. For my HO gauge layout. Silly thing was I didn't notice until the first one was half built and I suddenly thought, "That looks a bit big"
You live and hopefull learn.