I was thinking of making a guide for this, but since there is already a lot of YouTube videos on the topic, i'll just leave some of the good walkthroughs in here:
Police Cars:
Police Riot:
Jet:
Tank:
Buzzard:
I will recommend being
very cautious about this as it is considered exploiting a glitch in the game and may therefore result in an eventual reset/ban. Especially with the Jet, Tank and Buzzard, which is a CLEAR exploits to get an in-game advantage.
I had a Tank, a Jet AND a Buzzard in my garage 3 days ago, but to avoid the risk - and because the use of especially the jet and chopper is limited since it's next to impossible to get them delivered by the mechanic and they'll explode/get badly damaged when taken out of garage -
i've since removed all of them from my garage. I still have 3 police cars in my garage.
Here's a few shots from my own garage
Hope this helps those who wanted to know more about this.