Assuming you are talking of the most popular model the CZ-75 and variants (85/RAMI/P-01/SP-01)
Make the weapon safe, eject mag, rack slide, visual and or manual inspection of chamber, point pistol in safe direction, squeeze trigger.
Manually click hammer to the half cock position with thumb or forefinger and thumb (Optional but makes the next step easier)
Ease the slide back until it just stops (2 small lines on the rear of the slide and bottom receiver/pistol grip now line up) approx 1/4".
Use the edge/corner of the magazine to press the small nubbin side of the slide release through the slide (right hand side of the pistol if looking down from the top).
Remove slide release
Push slide of the pistol forward, off the pistol grip
Turn the slide upside down and ease the recoil spring and rod off the barrel.
Barrel will now be able to be retracted and drop free
Done
To re-assemble do the reverse.
REMEMBER not to push the slide past the two lined up notches when disassembling or you can't remove the slide release.
REMEMBER not to push the slide past the two lined up notches when re-assembling or you can force the slide release through without it engaging the barrel.
No tools required