These rules override the XP totals given in the individual class descriptions.
That means it will cost you a thousand XP to get from level one to level two, and likewise a thousand XP to get from level nineteen to level twenty.
The amount of XP awarded for any given situation will be entirely at the whim and will of a wise and benevolent GM.
XP will be awarded for overcoming perils, cleverness, bravery, moral fibre, and so forth. You might even get some for incredible luck. You're more likely to get XP if you're acting according to your class and/or alignment than otherwise.
There may be situations in which hard-earned XP are sucked away again, such as, for example, being fed on by a vampire, or putting on a cursed pair of boots, or any number of other unfortunate eventualities.
If you lose enough XP to go below Level One, then you will die. What happens thereafter is dependent on circumstance.
When training to advance in level, treasure can be substituted for XP at the rate of 10 gp per XP, up to a maximum of 500 XP.
If you have earned enough XP to be able to advance without spending any money on training, then you can. However, if you don't train formally, then you will not get any of the ancillary benefits: for example, the free spell a wizard gets when he or she trains, and your new rank will not be recognised within a guild or temple hierarchy. If you've already advanced in level, you might be able to get some of these benefits retroactively by paying a fee — or maybe not.
Players can spend their XP to modify their circumstances. For example, they could get a re-roll on a d20 or d100, or they might add 1d6 to an attack or damage roll.
XP spent in this way are gone, and it would be quite possible to spend yourself down to a lower level, or even, conceivably, to death.
NOTE: I haven't actually worked out the rates for these sorts of things yet. I'm leaning towards variable costs based on rolling dice of various sizes and numbers.