Here's what we want to make first :

We will use Sketchup because it's easy to learn and enough powerful for most usages. If you're confortable with another 2D/3D CAM, feel free to use it and follow along the instructions and dimensions with your own software. But you should give a try to Sketchup someday, it's very fast and efficient to prototype, even in 2D.

You want a shape that specially fits your phone dimensions. Grab a ruller and get the lenght, height and depth of your device in millimeter (or google it). Keep it next to you, you'll need it.
z) to zoom.
o to manually orient the view.
Use it with shift held to pan.
To simplify the export, we will draw everything on the ground surface :
Let's start drawing! Use the rectangle tool to create a rectangle with the dimensions : phoneLenght+20 x phoneWidth. My phone is 124mm x 59mm, so I want a 144x59mm rectangle. Don't go far 150x60.
You have to:
124;59

See the down-right field you type in.
Use guides to position the lenses holes :
32 and enter

You'll draw a circle from the center, with the radius of your lenses plus 1 millimeter for convenience. Your lenses have probalby a diameter of 34mm, fo you should use 18mm for radius
Del

We want space for our big nose! Use the tools line, circle, or arc to make a quick shape :


To create the sides, you've to keep in mind :
You start by drawing a template with the ruller, with the 3mm slot for assembly, 40mm space and 7mm slot for the phone.

Then add some space around

Use the rotate tool q to turn the top and bottom guide. Press alt while you do so to duplicate the guide. This works with the move tool m too!
You can use the 15° anchors or type any angle you want.

You can use the line tool l to draw on top of the guides.

Let's make a symetrie for the left side!
g. You just transformed your geometry into a component, which gets those behaviors:
m while holding alt.
s to flip it. You can type -1 or find the anchor.

You can apply the same trick to your center board:

Double click to enter a component, make the modification and see the other side updating. I used 3mm for the slot width and 5mm around.

You're alsmot done! You only miss some ring to hold the lenses in place. Use the circle tool c and Offset tool f.

Now you're done, and you can pass to the next stage to laser-cut your handset. If later on you want to use sketchup for 3d, here's some tips :
p. It's the core of sketchup.
g to avoid messing up your model.