The purpose of a reference architecture is to identify the architectural principles that apply when creating a sustainable and scalable solution. As new additions are made to the solution, the reference architecture becomes the yardstick that all solution proposals can be measured against and so enable a fair comparison of ideas.
A key test of the principles used to create an architectural model is whether they display conceptual integrity. Proposed solutions and additions to the solution must respect the conceptual integrity and show where they deviate from said principles as well as motivating why it was necessary to do so.
Deviating from these architectural principles is often (well-)motivated for reasons of time and cost when implementing the solution. However, without an explicit reference architecture one cannot measure the effect of these compromises, compromises that generally result in higher maintenance costs. And the larger the deviations from the reference architecture, the greater the risk for higher maintenance costs and the harder it will be to continue developing and scaling up the solution in future.
For over time, the tendency is always towards greater and greater divergence between the solution and the model; until finally the conceptual integrity of the solution can no longer be discerned. How then do we preserve the conceptual integrity? By training and information dissemination, what is known as shared understanding as preached in the Agile world. The development team must share the same picture of the reference architecture if it is going to be maintained going forward; and it will also be much easier to explain how a solution works if it adheres to architectural principles compared to one that does not.
An essential part of a reference architecture is the creation of architectural artefacts, such information models, state machines, process diagrams and so on. Using standard modelling notation such as UML and BPMN reduces the risk for ambiguity and makes knowledge sharing that much easier.
Where to start? Try to identify what type of solution it is you are building. Does it fit into a known pattern? Try to find the appropriate technical literature (in book format and online) that provides a frame of reference (including vocabulary), and use it to create a reference architecture which can then be applied to the solution.