1.) Read all component documentation; User Manuals/Guides.
2,.) Read again and highlight things that you are not sure about.
3.) Google and research answers to 2.) above.
4.) Write your own assembly plan to put it all together based on 1, 2, and 3 above.
5.) Set up a clean organized work space with lots of light and room.
6.) Double check that you have all the correct parts and pieces. Throw nothing away. Read any extra instructions sheets provided.
7.) Do not force anything: if too tight, not lined up, etc. then something is wrong.
8.) Then read the following to ensure that you are indeed ready to proceed:
https://www.tomshardware.com/reviews/how-to-build-a-pc,5867.html
Plan to take your time and do not rush - especially towards the end of the build. Take breaks - no need to use a marathon approach.