What do I need to do to start a genesis block and start to mine locally from lets say block 10,000 to setup a local bitcoin "fork"
For that, you need to run the bitcoind or bitcoin core with the regtest mode. A regtest mode is completely different from a testnet mode. With regtest, you can create your own blocks whenever you need and you will be having a very less difficulty as how satoshi mined the initial blocks. Note that a regtest application is no way related to testnet which is very much similar to the mainnet application.
Do the following to enter regtest mode :
Open Bitcoin Core GUI application. Then go to Settings -> Options -> Open Configuration File. Type regtest=1, save it and reopen the core application. In the console type
to generate 101 blocks and receive the 50 BTC which can be used for local testing purposes.