My prototype is done, how to go to the next stage?
I don't really know what you mean by open source hardware. This typically refers to a design for which schematics and gerber files are freely available from the vendor. Im going to assume you have bought and connected up several development kits, and would like to create a single pcb with all of the nessasary bits. 

If you want to produce a pcb, you should go through the following steps. It is not a trivial task, depending on the complexity of the circuit. 


Choose,obtain and learn to use a cad package. Altium or eagle or orcad etc. You want one targeted at electronics and pcb design. If any or all of your dev kits are available as cad package design data, it makes sense to choose the same package
Schematic capture. Copy the circuit diagrams for your dev kits into the cad package. You can just cut and paste and connect them up, but most likely you will spend a lot of time adjusting the circuits and removing bits you don't need.  Changing parts for cheaper or if they are unavailable to you. 
Layout stage. This is also a design exercise, though again if you have the design data available in your cad package you may be able to reuse much of it. Be aware that high speed / rf / smps layout is not trivial even if you have the source data. 
Mfr and assembly. You are now in a position to make a pcb. If your design is simple it can be done at home, but it's not that expensive to pay a fab house to make the board. Some will also populate it for a fee. If you have bga parts or very fine pitch you may struggle to hand solder the parts. 
debug and fix. No matter how much you check it, there will be mistakes. With any luck you can fix them without scrapping the board and making another one.