Building techniques for designing a LEGO City Street scene?
Calculations / Pre-process

I started off by guessing that your MOCs are about 7 minifigs tall.  If a minifig is 4 bricks tall that makes the overall height 28 bricks tall.
It looks like the tallest buildings are about 20 stories tall and the shorter buildings are 5 stories tall.
So you're looking at 4 plates or 5 plates per floor to build these.  4 plates per floor would end up with buildings 26.66 bricks tall.  5 plates per floor would lead to a 33.33 bricks tall building.  Since the 4 plate version is closer to our goal height, and we can cheat by making a few of the lower floors an extra plate tall, that seems like the best way to go.
Then I took the picture and made a diagram of where I expect the buildings to go.  Based on this it seemed like the building should be 12 or 15 studs per side.

Process
I realize this question was asked for LDD, but since it is no longer supported I'm not going to fight with it.  Stud.io or Mecabricks are actively maintained and supported.  I'm a bit better at Stud.io so I'm going to proceed with that.
For organizational purposes I'm going to put each chunk into a step and name the step something logical.  There is way too much in each step for LEGO standards, but I'm not worried about buildable instructions for now.  If instructions become an issue these steps should be split up further.

I laid out a couple of baseplates and created some boring sidewalks for the main road where the MOCs are stationed in the picture.
Then I added a brick ruler that is 28 bricks high so that I could conveniently have a visual reference for the goal height from step 1 under Calculations.  I did this with a stack of 2x4 bricks in alternating colors.
We needed more baseplates to go under the buildings so I added 2 more pn3811 32x32 baseplates.
I wanted to make sure the layout was going to transfer into bricks so I used 4x12 plates to layout the buildings from the sketch.  This will also serve as a &quot;basement&quot; to get the buildings to start above the sidewalk tiles.   I added building numbers that will come up again later.
I put in 2x4 tiles to make the side streets.  I went with Dark Bluish Gray to match the road baseplate color.  Since I'm giving the side streets 6 studs of width the remaining 1 stud width on each side could be filled in with a curb or sidewalk.  Filling in the road helped me realize I had a couple of buildings 1 stud off, so I was glad to catch and adjust that before it was more painful to fix.
There need to be something to tie the baseplates together so I put in a line of tiles 2 studs wide.
Whew, that's a lot of setup, but now we can get to working on one of the buildings.  I want to make sure the closest tall building looks decent.  I'm going to start by having a layer of plates that are inset by one stud and then a layer of bricks.  I wanted to do something SNOT for the vertical feature of that building, but I couldn't think of an easy way to float the 87087 &quot;Brick, Modified 1 x 1 with Stud on 1 Side&quot; out there.  I went with 1x4s jutting out and made it look like it goes up the whole building by filling in 1x4 plates.  Once I got a floor I wanted to go with I turned it into a submodel.  I then copied and pasted this submodel 19 more times.  Once you have the 20 floors you can finish the top in a few was.  I just through some plates on top to make it look uniform.
The next building down the street (&quot;building 2&quot;) presents more challenges.  I'm guessing it is 5 stores tall.  The top 3 floors are one style and the bottom 2 are another.  I'm guessing the bottom 2 are a parking garage.  There's a lot of light coming out of the front of this.  I'll leave the light issue as an exercise for the reader or a new question.  There wasn't much chance of doing repeating elements so I didn't create any subassemblies for this building.  The 27c01 windows aren't available in tan, but since this is all in the computer I'll cheat to keep it looking nice.
The last building on the main drag (building 3) is hard to tell much of anything in the picture.  I bricked up a boring cube to fill the space.  I turned two levels of bricks into a subassembly to make the repetition clearer.
The building behind building 2 (building 4) is obscured in the picture by building 1 so I filled in with another boring cube.
The building behind building 3 (building 5) is another 20 floor tall building.  I made 4 layers of 1-wide plates that looked like an interesting approximation of the picture.  I cheated again on some of the colors not being available in that plate size.  I made the floor into a subassembly and repeated it 19 more times.  I covered it with some plates.

Final Top View

Result

This isn't anything that I expect anyone to build.  It needs lots of refinement, but I won't take that joy away from you!  :) I hope this demonstrates the building techniques that you were looking for.  If anybody wants the file I'll find a place to post it.