asmtool or asmtoolg(Gui version) helps to stamp the new disks on windows for using as ASM Disks. You can use asmtoolg utility present in %ORACLE_HOME%bin directory which will provide GUI for stamping the disks.
As per Oracle docs
asmtoolg shows the devices available on the system. Unrecognized disks are labeled as a "Candidate device." Unformatted partitions are labeled as "Oracle raw device file." Stamped ASM disks are labeled as "Stamped ASM disk," and unstamped ASM disks are labeled as "Unstamped ASM disks." The tool also shows disks that are recognized by Windows as a file system (such as NTFS). These disks are not available for use as ASM disks, and cannot be selected.
You can find the above information and also steps to use asmtoolg at below link