Before you go ahead and try these jumps/routes/parkour in your own game; there are a few things to note/check, which will help make them easier to land and maximise your chances of successfully doing them :)

1- Upgrading your "Runner" perk as much as possible
This will make you run faster in-game without use of drugs which allows for bigger gap jumps. You might need to upgrade this once or a few more times in order to land some jumps or to make some relatively easy.

2- Not wear any bag
Wearing bags/backpacks will slow you down depending on the backpack and its content itself.
Wearing bigger sized & heavier bags will make you run slower and jump shorter which means jumps will be harder to hit/impossible.

3- Make sure you know what "Jump-Crouching" is and how to make use of it
Jump-Crouching (or Crouch Jumping) is a mechanic you will need to use to make some jumps possible. It will allow you to jump onto things/objects/structures higher than your jump height and/or make some jumps easier.
3.1- How to Jump-Crouch:
Jump-Crouching is crouching immediately after jumping; which will make your character/body in-game be placed higher in the air, and allows you to reach higher places than usual without jump-crouching.

To do this, you simply need to crouch right after jumping.
Note that in some instances, the surface you're trying to reach may be at an angle and you need to go past the slidy-angle by moving a bit/accumulating speed before making the initial jump.

4- You don't need drugs
If you're thinking about taking some cocaine, meth or AMP, just letting you know that will not help.
Taking these drugs will make you run faster, but also speed up the game; which makes it harder; and defeats the point.
If you're struggling with jumps; don't take drugs; try upgrading your Runner perk in-game or make sure not to wear any bag/backpack.

To get to the roof of the main apartment building, you'll need to make a series of jumps in order to get there, this is done by jumping from building to building (parkour)
Here is a simple video that shows you the correct route/way to access the roof:

Start from the alley way to the left of the west old town checkpoint, near the empty house; there's a broken piece of metal framing, Jump-Crouch on it and proceed to climb on said house.
Once you're on that house, make a jump to the side of the building next to it as shown in the video, then keep walking across its' ledge until you reach the other side (walk to the left)
There will be a flag-holder kind of metal pipe sticking out at an agle, keep crouching and jump on it; we make use of its' angle and climb up it; then jump on that building.
Next, locate the cloth-drying-wire-thingy sticking out on the building right in front of your main apartment building; jump on it by sprinting towards it and jump-crouching. Once you're on that cloth wire thingy, you can jump on the next one; which is on your apartment, it's right outside your window. Once you're on that one; you only have one jump between you and the roof. The easy way is shown in the video which is jumping directly onto the right building. The harder way is to jump Very Precisely on the one-pixel brick wall that's slightly sticking out of your main apartment building, you shouldn't try this at all as it's very difficulty and requires alot of skill. I landed this jump in my 1st try but that soon became clear that it was very lucky; as I tried for an hour after that to land the jump again but came to no avail.

To make some of these jumps (or maximise your chances of landing them) you will need to have your Runner Perk upgraded as much as possible so you can run faster, therefore land bigger gaps.
You should not be wearing any bag because it will make you slower/jump shorter depending on the bag and its' content & how full/heavy it is.

I'm sorry for being lazy but I already made a video that contains all/most of the jumps in a few minutes; and I'm not feeling like making 5 more short ones for each jump.
Enjoy!

If you'd like me to add more jumps/videos just let me know. You can contact me through commenting on this guide, my Youtube Channels, my Discord or adding me directly on steam/commenting on my profile. Please be nice and cheerful :D
I hope you enjoyed the guide and found it useful/fun; if so, please leave a like on the videos/rate the guide so more players enjoy messing with the cops/check out these cool places in the game!

If you'd like, I also make tutorials and funny videos on my DDS Youtube channel; here are some videos you should check out!