In order to build the perfect scene the first thing you must do is to decide what period and location you are interested in. This makes it easier in the fact that you will be much more likely to pay attention to detail if you actually have interest in the setting. Once you have done so then you need to make sure that your new model matches that setting and time period. It doesn't have to be so precise but if you are going for accuracy then you must ensure this. If it does not the you should consider using another engine or changing your design ideas.
Once you settle on your setting then you can do some further research into period buildings and structures so that you can make them fit into the scene. Once you know what setting and what buildings if any you will then need to choose a season of the year. You would not want to add flowers to a winter scene or lots of green hardwoods to a winter scene. Also you should try to find out what sort of trees and plants actually inhabit the area you are emulating.
If you are going the quick and easy route you can then purchase the trees, plants, and all that you need from a local hobby shop. If you can't find the ones you are looking for just ask a salesperson and they should be able to help you. Now that you have your indigenous species you can start adding your other features. There are a few different options available for this step as to which ones you will want to add. You may also be able to purchase these from the hobby shop as well.
Adding a train tunnel or bridge can add a bit of excitement to your model train display. You also will want to add a depot and possibly other features such as a granary or water silos. You can either add static props or if you are working with certain train models you can actually purchase working ones as well. Now you just need to spruce up your scene a bit. By adding in different things such as railroad crossings and people or cars you can make your model train display look a bit more active. You should also make sure to add inclines and slopes for your train to climb and go down. Make sure to vary the colors of your vegetation and use it as much as possible in conjunction with buildings, gravel, and roads so that the scenery is busy but not too much so.
Soon you will have an exciting scene set up and will be able to show off your new model train display to all your friends and family. Happy Building!