You need to think about the following things:

• How to validate
• Who will validate
• How to give incentive to the validators.
• Processing time.

Thanks.

A consensus mechanism is usually attempting to produce a solution for this problem, which is fundamental for distributed ledger systems.

https://medium.com/loom-network/understanding-blockchain-fundamentals-part-1-byzantine-fault-tolerance-245f46fe8419

You must remember that most networks have a series of nodes that are each competing or "working" for the ability to write to the chain. i.e produce the next block. Once a "winner" is found, all other nodes verify the solution, and then check whether transactions are valid. If they are, then they move onto attempting to find the next block.

So your consensus mechanism must ask for some "work" from nodes, and that work must be easy to verify by other nodes. the work done must also be variable, ie. it must be scalable to a sense of difficulty.

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Normally, we can find the complex of the problem and try to figure out the best solution for it.

There are several books on foundations of distributed systems that discuss the consensus problem, e.g. Cachin et al. "Introduction to Reliable and Secure Distributed Programming" https://www.springer.com/gp/book/9783642152597 .

I think, this is a good starting point.