First you need at least two medium (6-car) garages.
One of them will be the "enter" and another the "exit".
You can only have one "exit" garage, and it is the uppermost in your list of garages.
(needs confirmation)
Call Mechanic to see the list:

In my example "Procopio Dr" is the only "exit", but any of the other 4 can be used as "enter".
("Integrity Way" doesn't count because it's a high end, and this works only with mediums)
If you want to reoder the list, you have to trade in.

Second you need a convertible in your "enter" garage. Any of these should work:

• 9F
  
• Issi
  
• Zion
  
• Sentinel
  
• FelonGT
  
• RapidGT
  
• WindsorDrop
  
• Carbonizzare
  
• CognoscentiCabrio

As you probably noticed, animation for getting in convertibles is bugged, it makes you go half into the floor.
And that's how it works: if you go deep enough into the garage floor, you will teleport into your first garage.

To trigger the right animation - start getting in and then try to move away to stop the animation.

This method works in free roam and any mission where you can enter garage.