M/T takes time to learn if you have never owned anything with a M/T, like a Motorcycle.
Take time to be smooth, quick comes later.
Fortunately, the XJ is a great vehicle to learn M/T on as it has loads of low end torque!
For a smooth start, start in 1st with the clutch in, slip the clutch some to get moving and add gas as you release the clutch further. You can do this in 2nd, but it takes more slip (read wear), and more gas (not as efficient). 1st gear in a XJ is not a granny gear like a pickup 4spd, but more like an import 1st gear. Learn that and you can drive most current M/T.
Hill starts are a bit different. With your feet on the clutch and brake, let out the clutch slowly until you feel the Jeep start to pull on the brake, then release the brake and slowly add gas. The hand brake is there to help you do all three at once, but the XJ brakes are lousy at holding the Jeep from rolling backwards unless you constantly manually adjust them, so I gave you the two pedal version.
Downshifting takes more skill. This takes patience and learning your vehicle, but is worth it. To downshift, punch the clutch and pull the transmission out of the current gear, shortly after, pop the gas, and shift into 1 step lower gear. The idea is to get the new RPMs in sync with the gear to put the engine in the power band you want to either accelerate, or use as a brake. I very seldom use my brake except when coming to a stop.
Off-Road you don't tend to shift much and the game is all Clutch/Brake/Gas coordination. You will tax your skills there.
I drove MC for years before my 1st car (a 1200 Honda Civic). I drove around a lot around on different terrain to hone my skills with the new set of parameters.
As per above, time depends on the person. I taught my 1st wife how to drive a stick in 2hrs. My Brother after days, gave up and never bought another M/T vehicle.