KeshwaY

Use the "Stay Clam" mod.  I'll try and dig up a link...I may have found it at GTA5-mods.com
It does not interfere with LSPDFR and the peds, in and out of cars, will not freak out and will not start panicking - driving over the crime scenes and whatnot.  Works very well.
 
EDIT: Located here: http://www.lcpdfr.com/files/file/7969-keep-calm-beta-reduce-ai-panic-attacks-when-shots-are-fired/

We're proud to present the second major public release of LSPDFR, and by far the most epic version yet, LSPDFR 0.2.
LSPDFR Version 0.2 follows on from the groundbreaking success of our first release for GTA V, implementing a large number of features specifically asked for by our community.
You can find LSPDFR 0.2 on the brand new LSPDFR page, including a whole set of online resources and documentation, at http://www.lspdfr.com

I hate you now.

So your wanting to do like car or clothing?
First thing i would should suggest is you take an existing skin as reference maybe even put yours over it for placement ideas.
Second is Have something that can read .png files Photoshop should work fine or you can go a little more advance and get GIMP with the .dds plugin to read the .dds files
Third Have the most up to date version of OPEN IV. http://openiv.com
all vehicle files can be found in GTA V > x64e.rpf > levels > gta5 > vehicles.rpf
Idk where the clothing files are found becuase ive only done vehicle so far. Open the YTD of the vheicle you want to edit then select all or just certain ones you want to skin put them in a folder thats saved to desktop (call it GTA mods or w/e you like just so you know where your files are)
after youve edited/ made the skin u like replace the textures inside that ytd file. you can preview how it look by double clicking the the correspond ytf file or save rebuild the vehicle and spawning it in game.
i however and not sure beyond the police vehicls with editing the vehshar.ytd how to edit the skins on all cars. hope this helps

To follow on from the previous update I posted yesterday, we're still hard at work preparing LSPDFR 0.2 for release.  At this time, it would appear that all issues caused by the latest update to GTA V have been resolved.  Indeed, I spoke of the problem and its solution in more detail in the previous update regarding 0.2.
Unfortunately we're still experiencing issues with LSPDFR 0.2 which appear to be unrelated to the recent update and more of a problem with the modification itself.  These are in the form of script crashes that some, but not all, have encountered during pursuit testing and this is something we've been looking in to.  Unfortunately this seems to be quite a tricky problem to find as different users are experiencing different results.  Nonetheless, I'm hopeful that we'll find the problem and resolve it relatively soon.
While LMS has been hard at work debugging most of our issues (both the ones caused by Rockstar's protections and ones of our own making), I've taken advantage of the extra time to add more polish to LSPDFR 0.2, and even introduce a new feature which I know will be very popular.  To complement the police computer in a vehicle, here's a sneak peek of the equivalent system on foot:

Bear with us as we work this out.  I guarantee you that LSPDFR 0.2 won't disappoint.