First of all, you have to divide the layered structure. For the convenience of design, it is better to use the substrate as the center and distribute symmetrically to both sides, and the signal layers are separated by electric ground layers.
For the transmission line, the top layer is analyzed by the micros trip line model, and the inner signal layer is modeled by the strip line. The signal layers on both sides of the 6-layer/10-layer/14-layer/18-layer substrate are preferably simulated by software, which is troublesome.
The 6-layer/10-layer/14-layer/18-layer substrate is signal layer on both sides, and there is no electrical ground isolation. It is necessary to pay attention to the vertical alignment of the adjacent layer and avoid the AC loop.
If there are other power supplies, it is preferable to take a thick line at the signal layer and try not to divide the electric ground.
Next, ask the manufacturer for the parameters (dielectric constant, line width, copper thickness, plate thickness) for impedance matching. These parameters do not have to be calculated by themselves and should be provided by the manufacturer. With these parameters, you can calculate the line width, line spacing (3W), line length, then you can start drawing the board.
The high-speed line is best to go inside, and the top layer is easily affected by external temperature, humidity, and air, and is not stable. If you need to test, you can test the via lead. Don't have the illusion of flying lines and secant lines. Multilayer boards don't need "hands-on ability" because the lines are inside and high frequency, they can't fly, and the lines are very dense and can't be drilled.