Here are some easy ways to get a Lazer Jet in Fort Zancudo, which are usually fail-proof if done correctly.
If using a helicopter:
First, hover it over this direction. Sometimes, if the Jet isn't parked there, swing your camera around or reverse cam your player and it will appear on the ground.
Now you can either:
A. Rappel down with a Merryweather Helicopter (takes a little practice to find the right spot to rappel down) Merryweather Copters have immunity from wanted stars upon entry to Fort Zancudo. It's only when you land on foot will you get a 4-star wanted level.
B. Land a helicopter next to the Jet (which is more risky since there is a high chance that a guard will be next to the Lazer which you must kill)
Immediately get inside the Jet. Once inside any soldiers that got outside to shoot you see you getting inside the jet, will get back inside their jeeps or trucks because they have the same A.I as the police then you speed off with a car.
This will prevent them from shooting you on-foot and just chase you slowly with their cars while you are inside the jet.
Now, turn the jet around, facing the security fence over to the Lago Swamps. Now you have to speed up on the ground and then fly low, up and over the fence and either:
A. Have a phone call with Lester ready to hit the "remove wanted stars"
B. Fly very low on the swamps and past the bridge to avoid an enemy jet to lock-on to you or spawn in the air. Then you can gain altitude high above the sky and wait for your wanted stars to automatically remove. Flying very low prevents any NPC Lazers to spawn and lock onto you in the air.
My favorite way to get inside Fort Zancudo, and is basically easy to do for any level is with a bike or car (calling a taxi works) and finding this hill slope on the other end of the highway tunnel at Fort Zancudo.
If you cut to cinematic cam, it will increase the odds of messing up NPC character/vehicle spawns around the area making you get a clean break:
In the rare chance that a tank does spawn in the area, which rarely never happens -- here's an easy way to get revenge on a NPC tank.
They aren't as mobile and trigger happy as regular players apparently. I could of quickly stuck 3 sticky bombs on their weak spot vents to blow the tank up, but I wanted the guy to have a taste of his own medicine