Once purchased, you can download the deeptree libraries of your choice, containing the assets in collections.

All textures are packed into the blend files and can be unpacked from there, for inspection and modification.
You can append the tree assets into your scene via file/append.

Choose the collection from the collections folder.

SETUP

Once loaded, you can either place them directly or use the collection in a particle system.

Emission should start  and end at frame 1.
And their lifetime should match your timeline.

If you use a particle system be sure to uncheck Object rotation, and activate rotation, to either align particles to the normal or the global z axis. That way you can set randomize phase to 2, in order to rotate all particles aournd their up axis.

Particles need their local y-axis to be the one pointing up. So all assets are rotated around x already, for their local y axis to point to the global z axis.

Use randomize phase to rotate the trees around their vertical axis,
to maximize variance.

Set physics to "None"

Set render as "collection" and select the collection. 
Don't use the rotation, otherwise the particles might lay on the ground, instead of pointing up.

Each tree can be tinted in a dedicated mixRGB node,
that's connected to the base color of the shader.

If "overlay" not sufficient you can try other blend modes, like "color" or "multiply".

Avoid close-ups of tree assets, they work best at a distance

MODIFY YOUR TREES

cycles is more demanding with fake-depth shaders. The textures are not rendered as contrastful, bump-maps are antialiased much better (making smoother results) and the shadow ray terminator shows hard lines between lit and shady polygons.

   
cycles                                  eevee

you see the trees blend much better into each other in eevee, while cycles reveals more of the low poly geometry in the shading, resulting in the unpleasant flat look.
What could you do about it: 

Adjust the light shading-shadow terminator of the tree in cycles:

This should smoothen the visible edge between lit and dark polygons of the tree.

Next you can add modifiers to increase detail in the mesh:

without modifiers:

with subdivision (no shading edges):

with subdivision and displace (using a clouds texture):



here are the settings used for the modifiers: