Giant blue stars burn all of the way up to the most stable element of all, which is iron. And then collects an iron core. As the core grows, the star shrinks and the pressure in the middle grows. Once the pressure in the middle is high enough, it overcomes the Pauli exclusion principle and electrons merge with protons to become neutrons with a release of energy, which mostly comes out as neutrinos.

Once a bit of that core disappears, the stuff around falls into the new gap, hits it, goes under way more pressure and the same thing happens. This starts a chain reaction that causes a supernova. The core collapses into a neutron star while the release of energy blows the outer shell out to eventually become a nebulae.

If the initial star was big enough, the newly created neutron star is big enough to transform into a black hole. Otherwise it is left as a neutron star.

If it forms a neutron star, the spin speeds up thanks to the collapse. Typical time to rotate can be anywhere from milliseconds to seconds. We can tell that because its magnetic field also gets trapped and forms a strong beam along the north and south magnetic poles. Since magnetic poles do not generally line up with the rotational poles, that beam becomes like a searchlight. If we're in the path of that beam, we see a pulsar, and the frequency of the pulsar is the rotational speed of the neutron star.

This is probably way more than you wanted to know. :-)

I probably also am going to get corrected on some details in 3, 2, 1...