The pcb files are in zips with the contents in a format called gerber which can be uploaded to a pcb manufacturer like jlcpcb. The gebers contain all the necessary information for the creating the copper tracks, drilling of holes, silkscreen printing etc. Often there are minimum order sizes of around 5 of each type of board so often people have spares...
There are no step by step instructions. There is an assumption that you can read a schematic, are prepared to read through some very lengthy forum threads and do basic drilling for mounting boards to heatsinks etc. the information is a bit spread-out.
In general you need:
BOM - bill of materials to get the right value and size components. Downloads from the appropriate forum thread for your build (and possibly two threads - the second thread being for the power supply)
schematics - for troubleshooting etc in the forum threads and or download from https://drive.google.com/drive/folders/0B7egryukiT7_TFlEQlBRejdVdDQ
the gerber files for getting the pcbs made in the forum threads or download from https://drive.google.com/drive/folders/0B_iJFfZStuVhSE5nOHBVdTByR1k
the specifications for the transformers. in the forum threads.
advice. in the forum threads and ask in the threads.
in general the markings on the pcbs takes precedence over the schematic or bill of materials (unless the pcb has a silkscreen error).
Some amps have more information than others - the threads grow organically over time depending upon popularity, etc.
In general support is available for everything on a voluntary basis. You post questions on the appropriate thread hope for someone to answer. There are some very knowledgeable and helpful people active in the forum for example Kevin Gilmore, Kerry, JoaMat. Often the people who created the design will answer questions in the forum posts or people who have actually built one or even modified one.
Unless you have lots of money/luck/ or existing stocks of obsolete components obtained from known good sources, I would stay away from any design which requires no longer manufactured transistors. There are many fakes floating around which can and will fail catastrophically on first switch on and non fakes are rare and there are few trusted sources willing to sell to non friends.
I would advise reading the forum threads from start to finish and making notes, that way you can see the different versions/modifications emerge, see issues with certain components for example in the DIY T2 thread the voltages are high enough that some makes of resistor sparked and the insulation broke down.... Look at what people did that worked or did not work and learn from their experience.
Finally if you can, give as well as receive. Post what works for you, post pictures, post updated bills of materials etc.