There are some absolute masters of PCB design on this site, I am far below that level, so take this all with a heap of salt. A lot of what follows is generally good advice but not everything is universally applicable.
Basics: learn to use your EDA software, properly configure it with your board house's capabilities, get correct footprints, read and re-read and re-re-read the datasheets for everything you use. Study other similar designs and try to understand everything they're doing and _why_.
- Place mounting holes and critical components first. Tiny boards and tiny components look bigger on-screen, zoom out to 1:1 real life scale as a sanity check!
- Use as many of the largest decoupling caps you can get. You don’t need multiple caps in different sizes; this comes from the old days of leaded caps when parasitics would be bad
- For power: use planes when possible; use a trace width calculator; always have a ground plane.
- Generally speaking, use the widest traces you can.
- There is a huge asterisk on this one, but most traces should be made as short as possible. Decoupling caps should be super close to where they're needed. This is one of the more common noob mistakes, but it can also lead you astray (making overly complex or compact PCBs on the first try.)
- Do not put capacitors or inductors close to the edges of a board, they will fail because of flexing!
- Check clearance between parts for pick and place and hand-soldered parts
- Always run DRC checks (there are also secondary DRC check tool websites/downloads aside from the one in your EDA software)
- Before sending it off, manually check for obvious common blunders (forgot the ground plane, no copper pour on ground plane, dead short, forgot to drill holes, wrong units, used the wrong footprints) - manually measure a few things on your design including footprints and pad sizes and cross reference this with an independent source. Check your files in different gerber viewers and hand-trace through the copper path from one component to the next. Visually preview the PCB and ensure you're not missing any copper anywhere.
- Don’t make things as small as possible right away! Make it big, test points, connectors, break out sketchy features into daughterboards etc, then shrink when it works
Beyond the basics:
- Understand your components. There are countless types of resistors and capacitors, to say nothing of the other component types. Getting more advanced, try to understand the various types, their lifespans, failure modes, heat tolerance. Pay attention to physical component sizes, if some capacitors of type X and rating Y are one volume and the others are half the volume by being half the height... why?
- Understand heat. For the most basic calculations: "With only natural convection (i.e. no airflow), and no heat sink, a typical two sided PCB with solid copper fills on both sides, needs at least 15.29 cm2/2.37 in2 of area to dissipate 1 watt of power for a 40°C rise in temperature. Adding airflow can typically reduce this size requirement by up to half. To reduce board area further a heat sink will be required." - from Thermal Design By Insight, Not Hindsight by Marc Davis-Marsh
- Get a better understanding of electricity and RF in general. This really pays dividends in terms of understanding why the "rules" are what they are.
For some interesting stuff beyond the basics, or to get yourself thinking, these links are great:
https://resources.altium.com/p/2-the-extreme-importance-of-p... by Rick Hartley
https://codeinsecurity.wordpress.com/2025/01/25/proper-decou... by Graham Sutherland
The "PCB Review" threads on r/PrintedCircuitBoard are great places to learn as well.
Beyond that... well, it's like any skill, learning the theory and best practices is great but the way to really improve is to get out there and look at (and design) tons of PCBs.