My current hurdle.
Searching the library for parts.
My relay does not exist in the library. Do I build one and add it?
yes.
I'm assuming you are using through hole components and will be working on a standard 0.100"x 0.100" track grid.
My IC's are in sockets. I dont think that makes a difference.
it will make a difference on how close to other components the IC can be placed.
A socket takes more area. Look at the "prolib" PCB symbol library...DIP8, DIP14
I'm using the discrete library to select caps, diodes, and resistors but I'm not sure if they are physically the right size. Mainly the caps. They are all different sizes.
There are pcb symbols named "DISC04' and "DIOD04" in the discrete library that can be used for all 1/4 resistors and 1N4148 diodes. The pads are spaced 0.400" apart. I typically use 0.200 pad spacing for 0.1u-0.47u caps, but you'll need to check the datasheet for the caps you use.
Try to pick a cap spacing, on 0.100" centers that will work for most (or all) caps in you design.
I'm using "header" to indicate the inputs but some of my pins are 2.54mm apart and others are 5mm. Not sure how to be sure that the pins i've selected are the correct dimensions.
You'll need to choose a part and look at its datasheet. You'll be working with close tolerances, so you'll need to check the footprint on the datasheet against the library footprint you use.
Look at parts available from your supplier, choose one, then find a library part (or make one).
Keep any parts you make in a custom library named, for example, "mylibrary".
Its a good idea to start with a component list, find the real parts, check/make/verify library parts.
Once thats done, create/update schematic, then create PCB design.
Try to ensure your schematic is correct before starting PCB design, although, as your design progresses, you will make multiple iterations for changes that are made.
Some advice, if you have to make circuit changes, always change the schematic and forward the change to the PCB design (if the PCB design exists). The design process moves more smoothly that way.