Yeah it is a stupid question but how do I do them? Sometimes I just can't predict what kind of jump it will be next, they are random for me. Is there a technique to jump higher all the time?

Here are a few tips and tricks:

Hold the jump key at least until you hit the peak of your jump. Letting it go early will make you hop.

Getting a running start. A width of about 1 or 2 character widths is enough to get full (running) speed. If you can hop for a while, you can gain lots more speed (which is unnecessary to finish the game.) You can clear a Mega Anneludine this way, or also the way stated next too:

Here's a small glitch: Hit JUMP and Q at the same time, without any boots equipped. In the menu, equip your Winged Boots then return to the game. Make sure you're holding JUMP when you get back out, so you can jump the highest. You will spring high up, high enough to escape Radkelu's pit.

edit: The reason this works is because when you have winged boots on, your jump strength is reduced but your jump gravity is also reduced. If you jump and immediately switch the winged boots, you get the jump strength of no-boots and the gravity of winged boots, which is why you go so high.

~Jordan

I keep dying to the inconsistency of jumping :( It's making me give up Ardentryst after quite enjoying the first 10 minutes.

The jumping isn't inconsistent, but there is a big variation between tapping JUMP and holding JUMP. For best results always try holding it