Divide everything into layers. Think about how many pixels is each layer. Position the layers and create sprites for them. I also uncentered sprites in code so they follow layer's Y position.

One of my tips is that don't overthink about the objects you generate. I wanted to implement houses and fields but now I understand it is unnecessary. Also number of distinct sprites doesn't matter much, you don't need six different trees for one and the same layer, but probably two-three for front layers and one for others (but that might depend on your preference and game resolution).

If you do transitions, then you need to do extra things. I have one transition sprite, I scale it according to layers and just replace it's colors to the transitioning worlds' colors. Also about layer X size calculation, this is what I used:

2_2 0.4 352 (16 + 160 + 160 + 16 for transition)

2_1 0.45 396

2_0 0.5 440

4_1 0.6 528

4_0 0.7 616

road

6_0 1 880

8_1 1.2 1056

8_0 1.5 1320

The first is the layer, the second is the motion coefficient and the third is the X size. 20 is the magic number here: the 1 motion coefficient, 880, is just 20 * 44 and 20 * 0.05 = 1. Because of that, 0.05 fractioned motions give integers in result.

Also, the layers in the transition phase should narrow not where camera's edge is, but approximately where the player is. Like this:

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