Do you have a grate layout for map but you don't know where it can fit in a real word place ?

First you have to look at your map , is it trying to simulate a more open area or is it indoors.

If you consider that your map can be set somewhere outdoors , first thing to consider is location in a real word continent .
+ For example you can chose Europe or America for a more urban fell , Asia for a mountain like area or ocean levels .

It is very important to have a very general idea of where the map can fit .
I will chose Europe in this tutorial because it has many countries with a lot of cities so you can get inspired.
+United Kingdom has lots of tall buildings that bound to each other and usually there is fog
+Italy has some nice open areas , lots of small markets an stunning sea vistas
+Germany is a more urban that the others but is has some nice vintage buildings

For this tutorial I will choose Italy because you can really impress a player with beautiful level and a interesting way of integrating in within the level.

After you choose a country , state or a location that fits your map , you have to make research about it . You will start searching for pictures of an area , types of furniture , architecture , weather , etc ...
This is a very important step because you want to make a very believable and appealing area.

Once you got all the informations that you need , you start integrating areas that you saw in your research in your map.
+For example I gonna put a tower in the middle , in the left side i am gonna put the vista , and in the right I will put an elevation and some houses for making it larger than it actually is .
The next step is adding textures , props , lighting . From here this is the part where is it better to stop looking at any picture and make the world yours .

Let's say that you are gonna choose the same examples that i gave , every world will turn differently , this really shows your creativity and perception of the world .

If you want to make an indoor area you do the same , search for house interior , look at the pictures , integrate the location in your map , add props and make it yours . Lighting is very important here so you might want to watch some lighting tutotrials .

Keep this is mind BE CREATIVE !!!

Thank you very much for reading this , i hope you learned something new for this read . For those how still have question , fell free to ask my on my profile . Have a nice day !

Note: I have used the Black Mesa Source Beta Photos because it really shows what can be done within Source , but I have no right for using .