PcPartPicker is a wonderful site that allows people to easily put together a build and get aggregated (collected) data on parts pricing, however there are a few important features that are not explained. I hope to walk you through them here. Firstly we're at the index page, nothing particularly useful except for the 'VIEW YOUR System Build' button in the top left hand corner. This is how we start a new build.
Your page should look something like that, if it doesn't press the big blue 'Start A New Part List' in the upper left hand corner of the screen. (From top to bottom) there are two important things to point out: The 'Choose a X' buttons and 'Add Custom Part'. Custom parts are handy for when you want to include something outside of what pcpartpicker contains (an old graphics card for example).
You should be met with a pop up box, pretty self explanatory. If you have a URL to a part (https://www.amazon.com/Gigabyte-GC-WB867D-I-Bluetooth-Frequency-Compatible/dp/B019JH89YI for example) you can add it in and press 'Create Part'.
Press 'Add To Part List'.
Now its been added in, easy. Pcpartpicker isn't perfect however, sometimes we need to specify the details ourselves. Press 'Add A Custom Part' again then press 'Add Custom Part'.
From here we can fill in the details we want as well as select a category.
Now we go back and add a motherboard. Notice on the left the filters and on the right the 'Add From Filter' button, together these are 'Parametric Filters'. Parametric Filters allows Pcpartpicker to semi intelligently choose parts based on what you select (as well as considering compatibility), then it will select the lowest priced part. This is one of the sites most powerful features, you can also select individual parts using the checkbox to the left of their name.
Here we select the 'LGA1151' socket, DDR4 RAM and 4 RAM Slots.
Rinse and repeat, pcpartpicker won't let you add in parts that are incompatible with each other normally. For the sake of time I have filled out the parts. It's important to note the price at the bottom, because we were smart and used parametrics the list will update to reflect current pricing, combos and will even update to different regions!
There are a few features to note here (from left to right): The Permalink which is a direct link to your parts list so that you can share it with other people (important if you want help!), the system wattage estimation and the 'Save As' button (for when you're using an account).
That's about all there is too it, happy part picking!