Simulation, simulation, and simulation.
any suggestion on software simulation?
For my work I use "professional" tools, by which I mean tools that are unfortunately outside of the budget of the average hobbyist.
For full 3D simulation we use HFSS - transitions from high metal layers to the gate/drain/source connections of the transistors on chip. Full verification of system before tapeout but these are simulations that take days or weeks. Non-planar structures such as wave guide launchers, SIW, etc.
General RF design is in Keysight ADS/Momentum. This is also the tool where we put everything together (either this or in Cadence Virtuoso). Things we use this for would be on-chip inductors and transformers during the design (once we have a design that seems to work, we use HFSS to do a full 3D simulation to make sure, but this takes a lot longer). I've also used it for distributed filters and matching networks. This is the core tool I use for doing PCB transmission lines - We would put a rough idea of the transmission line in Altium or Allegro, and then export the design and put the actual line in with ADS so we can verify and simulate performance.
I have little experience with "free" tools, though I hear a lot of people talk about QUCS. I've never used it myself, so I don't know if it is actually any good. The pictures I've seen look like it is going after a free version of ADS.