Hi!
Welcome to the CoG writing and readers community forum. Most of us are both writers and readers, so you’ll see both open WiP (Work in Progress) threads from us and us volunteering as beta readers.
There are two different beta processes - the first is the official process if you are writing a story under the CoG label - that is covered in the contract and such that CoG would send out to you but basically, your project manager will set it up and run it for you. @Gower is a published author here, so he can go into more details of this beta process then I, who is writing for the contest or publishing under the HG label like @Scribblesome is.
For most of us, setting up a “public” thread for basic communication in the sub forum HG WiP is the first step. In this thread you can present your project for the initial feedback most of desire before green-lighting it. Some of our hopeful authors are on this stage of their projects.
The next step is the step @Scribblesome and I are on and that is presenting your green-lit project to a group of beta readers. Scribblesome has chosen to present a public “demo” that they first put on Dashingdon web hosting - a free service for the community (a treasure really) and which they update with regular rewrites, revisions or with new material.
Another approach, which is the one I took, is to solicit volunteers for a private beta session(s) which you open a private thread involving only those you invite to it. You then present your work and have the volunteers beta it.
In both of the above cases, you can ask for what CoG classifies as “low-level” feedback and “high-level” feedback. Low level feedback is the English 101 feedback, including: editing, grammar and such things as structure and spelling. High level feedback is the stuff @Gower mentions: the continuity, character development, mechanics and such.
The public threads can stay open as long as you are actively pursuing the project (they automatically close if the project has no activity for three months) and are ready to submit the game for publication. I think our record is the recently published Choice of Rebels by @Havenstone at 6 years. He is an advocate for the open public testing and can answer most anything asked.
The private threads stay open until you close them or “archive” them. I suggest keeping up on the housecleaning aspects if you go this route - having multiple private testing threads open can be confusing - I speak from experience here.
Now that I given you a wall of text I will end by saying Welcome once more. We all look forward to your project. I also have some links for beginners I can provide if you desire them.