How to set up a private chain/local testnet : reverse engineering the blockchain code
The question slightly exceeds capabilities of this q&amp;a forum. Maybe better to search bitcointalk.org, there these type of requests get often discussed. Join one of the discussions there!

Anyhow, we need developers, and people with fresh minds. 
Here my short shot at bitcoin, because we are in bitcoin q&amp;a forum. You would probably want to create an altcoin of it. For sure the relevant info and link to the code is on bitcoin.org.

To accomplish your goals, you would have to have an idea and sometimes excessive knowledge on the following:

Information Security
Overview of current crypto coin technologies
Finite Fields
Elliptic Curves
Elliptic Curve Cryptography
Signing/Verifying Messages
Parsing Transactions
Signing Transactions
Creating Transactions
Script parsing and processing
Address construction/Private Key WIF Format Construction
Extended Keys (XPriv and XPub)
Parsing Blocks
Validating Proof-of-Work
Difficulty Calculations
Merkle Tree Construction 
Merkle Proofs
Network Message Parsing


and maybe 

Segwit
Lightning
Altcoin
... 


The list can be for sure extended :-) So you choose to go for a steep learning curve. From what I read in your message, I think you are not (yet) there, but that's not an issue. You will probably want to read books like Andreas' "Mastering Bitcoin", and some more. They will bring you up to speed.

If you think of on the environments (like Ethereum or IOTA), I doubt that many devs are here to support. 


  1) decide which existing blockchain would resemble the most with what
  i have in mind in terms of consensus algorithm


this is s.th. we can't help you with, as you haven't presented anything. Do you want to use the community's power &amp; efforts to bring your project forward, but don't provide input to the community by hiding your logic and ideas? In general: we are very curious here for new ideas :-)


  2) download if from github


see the links on bitcoin.org


  3) establish a dev environment


You take a linux machine, and install the software, compile it, and see how you go. Alternativly you can try your luck in other OS, but support is way better on unixoide systems. Also the usage of virtual machines is very attractive. 


  4) run different tests and let my brain do it’s magic


again, this is up to your mind howto. But there are testcases deliverd with the software, so you can find relevant code for it, adopt it to your requirements, and go.