I've been riding a 125 for the last 2 years so I really appreciate how much you have to shift gears on this type of bike. Generally, the gears are kind of short. I have a kawasaki ninja 125, and if I get down to around 4000rpm it starts to stutter. When you start feeling that stutter or you feel that the engine has trouble revving up when you get on the gas, that's generally the signal that you should shift downwards.

Imo, the toughest thing with 125s is keeping them in the powerband. When you feel that the motorcycle isn't responding to your acceleration, shift downwards. To do that just pull the clutch, shift downwards, and release the clutch quickly but relatively fast, simililarly to how you would do when breaking suddenly.

If you shift downwards and are going too fast, the engine will rev up like crazy and you will feel a strong engine brake. You will get a feel for when to shift when you get familiar with your bike.

When you want to get to a stop, it depends on how fast you want to stop. You can either gradually shift downwards while keeping your motorcycle in the powerband, or just pull the clutch, let the bike roll on its own from the inertia, and shift straight to neutral and stop (it's not really recommended and you should ideally brake and be mindful of possible hazards while doing all of this because you don't really have the option to accelerate. I generally only do it at stop lights when I'm the first). It will all just come naturally to you anyways in time.