Cheers!
First the models were built in LEGO Digital Designer, then I used @M2m's converter to bring them into Blender where I posed them.
I set up custom materials for the models using the Principled BSDF shader, including a tiny bit of subsurface scattering which breathes a little life into the bricks.
Then the meshes were edited a bit, mainly to add bevels to the edges for realism.
The ground is a tiled texture from Poliigon which I sculpted large lumps into. For the finer details I used a displacement map, which modifies the mesh on a much more precise level.
For the foliage I placed a bunch of objects in LDD, converted them and set up the materials. Then it was a simple case of duplicating them around the scene with slight rotations.
I always use an HDRi for lighting and usually some additional lights too, but for this scene the environment lighting was enough. The camera is set up roughly following basic composition techniques like the rule of thirds. A focal point is set with an aperture for depth-of-field as well as a focal length to get a dynamic shot. I considered adding motion blur but decided that to fit the theme of a child taking a photo, it wouldn't make sense, which was great anyway as it's less work for me
After countless test renders and constant tweaking to until I was mostly happy (there comes a point where you have to kick out the perfectionist inside!), I sent it to a render farm which spits out a number of different passes that I can use in post-processing.
After that a ton of further work is done in Photoshop. Corrections such as fixing "fireflies" (bright white pixels in the render) come first, then I paint in smoke/dust and add mist. Next I use the Nik Collection plugin to do a bunch of colour work which really transforms the image - while it's best to get colours, lighting, etc in the initial render looking as close to the final piece as possible, there's always more that can be done in Photoshop and it's sometimes easier. Finally I'll add a vignette and camera imperfections: film grain, lens distortion and chromatic aberration. Funny enough they're things that real photographers work hard to remove from photos, while CG artists do the opposite to try and get their art looking more realistic haha.
Hopefully that was somewhat helpful? Blender can be a bit daunting, but the latest 2.8 update included a huge change to the interface and it's more accessible to new users now.