A step-by-step explanation would be very lengthy and likely to go astray at any given step in the process.
Physical space matters, available connections (ports/plugs) matter, and most importantly - the motherboard must support the components that you wish to install.
Overall, you must be very sure about how everything is put together by reading the installation manuals and working out the steps that you must follow - one step at a time. Make sure that all components, connections, slot assignments (CPU, GPU,RAM, etc.) are supported by the motherboard.
You must use the applicable User Guides/Manuals for your case, your motherboard, the PSU, and other components being installed.
The motherboard manual is the most important document and likely to provide the most "how to" information with respect to installing the components.
The PSU wattage is determined by adding up the wattages of all components and adding 40%.
There are online calculators to help make such calculations. You should use two or three online calculators to determine a consensus value for the wattage required to fully support your build.
After establishing the needed wattage then research PSU's. Read product reviews such as those found in this Forum.
Read carefully. Details matter.