What are the steps for a beginner to electronics to get custom PCBs made for a simple adapter?
Step 1: Obtain a PCB design software and get familiar with it. Free options include CadSoft Eagle Light Edition and KiCAD EDA. Many others exist, but these two might be easiest to find basic tutorials for.

Step 2: Find the footprints for the connectors involved from the respective datasheets, or, if the manufacturers provide downloadable footprint files for your selected software, use those.

Step 3: Create a PCB layout with the two connectors, and PCB traces running between respective pins on the two footprints from above. Keep note of mirroring of footprints / pin numbers due to the connector being on the upper or lower layer of the PCB.

Step 4: Convert the PCB layout to Gerber format and order the PCBs from one of the many small-order PCB services, such as ITeadStudio, OshPark or other prototyping / hobbyist friendly PCB services. Personal opinion: Those two work out the cheapest for small orders of small boards.

Wait for delivery, solder the connectors, and offer the spare boards to whoever wants them.