Go here and screw around with the genesis tx and nonce. You probably also want to change other stuff (network id) to avoid contacting live Monero nodes.

I've helpfully decoded the genesis tx so you know/can guess what to change:

version 01 unlock time (varint, height, 60 here) 3c vin length (value in) 01 vin #1 (of 1) type (gen, 0xff) ff height for gen input 00 vout length (value out) 01 output #1 (of 1) amount (17592186044415 as varint) ffffffffffff03 output #1 type (to key, 0x02) 02 output #1 key (32 bytes) 9b2e4c0281c0b02e7c53291a94d1d0cbff8883f8024f5142ee494ffbbd088071 extra length in bytes (varint, here 33) 21 extra pubkey tag (0x01) 01 transaction pubkey (32 bytes) 7767aafcde9be00dcfd098715ebcf7f410daebc582fda69d24a28e9d0bc890d1

Go here and screw around with the genesis tx and nonce. You probably also want to change other stuff (network id) to avoid contacting live Monero nodes.

I've helpfully decoded the genesis tx so you know/can guess what to change:

version
01
unlock time (varint, height, 60 here)
3c
vin length (value in)
01
vin #1 (of 1) type (gen, 0xff)
ff
height for gen input
00
vout length (value out)
01
output #1 (of 1) amount (17592186044415 as varint)
ffffffffffff03
output #1 type (to key, 0x02)
02
output #1 key (32 bytes)
9b2e4c0281c0b02e7c53291a94d1d0cbff8883f8024f5142ee494ffbbd088071
extra length in bytes (varint, here 33)
21
extra pubkey tag (0x01)
01
transaction pubkey (32 bytes)
7767aafcde9be00dcfd098715ebcf7f410daebc582fda69d24a28e9d0bc890d1