Selecting the right DC DC converters

I'd like to select the right DC DC converters and just want to make sure I am on the right track. The board will have 3.3V, 5V, 12V, and 24V supplies. My plan is to use a 24V supply and then step it down to 12V then down to 5V then down to 3.3V. The specs I usually look at is the output wattage first and then look at efficiency and so on. But my main concern is the wattage and whether I am making the right assumptions when selecting the right amount of output power. From my understanding of power calculation theory, if I have a load that draws 1A at 5V, then my 24V supply will draw 0.208A. I found that value by calculating the power absorbed by the load (5V*1A = 5W), then calculated the current sourced from the 24V (5W/24V = .208A). I'm assuming that if 5W of power is absorbed by the load, then 5W would be delivered by the 24V source. Am I making the right assumption?

Yes u are right, Theoretically. But u can't just give strict 0.208A and expect the system to work, u have to add some margin which is consumed by the parasitic effects usually called as the drop out of the regulator that u'll choose for the job.

Give description of the topology that u'll be using to generate all those voltages like below:
1 - 24V -> 12V -> 5V -> 3.3V
or
2 - 24V -> 12V
24V -> 5V
24V -> 3.3V
U have to be careful if u choose the first approach.
Moreover there are lot of specs that u still need to consider depending on your system and its complexity, like whether the system is Analog or Digital or mixed, susceptibility to noise etc... if its a complex system, then describe it.

If the total power is only 5 watts, the easiest (on paper) is a multi-tapped flyback.

Having said that, the multi-tapped transformer will be a custom-made component. Transformer fabricators will want a minimum quantity (perhaps several hundred) to make it for you.

Otherwise, it is best to use three bucks to generate the 12, 5 and 3.3 from 24 V.

If you need tight regulation on any of the supplies, then it should be independent of middleman supplies. It should not be at the end of a cascade.

Conversely, if you will vary the load suddenly on any of the supplies, then you should not make it a middleman to another supply.

Thank you guys for all of your responses!

how you do this (top post) depends on the power drawn at each voltage, and also on the tightness of regulation required at each rail. You can actually get pwm controllers which provide dual buck converters and eg give a 5v and 3v3 output from a 12v rail, say. linear.com do them.