ASM instance creation in RAC, if ora.asm resouce is already exists

1. You may need to create an init file under GRID_HOME/dbs directory with below basic details.

/*
asm_diskgroups= <list of diskgroups>
asm_diskstring= '/dev/oracleasm/disks/*'
instance_type='asm'
large_pool_size=12M
*/

later you can create an spfile from pfile.

2. Enable ASM instance Auto Start as grid or oracle OS user (as the Grid Infrastructure OS owner) as follow:
crsctl modify resource "ora.asm" -attr "AUTO_START=1"

3. If you have created ASM disks then make sure the disks are discovered by kfod as grid or oracle OS user (as the Grid Infrastructure OS owner) as follow:

GRID_HOME/bin/kfod asm_diskstring='ORCL:*' disks=all

or

GRID_HOME/bin/kfod asm_diskstring='<full path ASM disks location>/*' disks=all

4. Once after creating spfile, you can update ASM with SPFILE path.

srvctl modify asm -p <ASM spfile full path>