As Gokul notes, creating vsd groups is optional but it is an important choice whether or not you use them.
Without the vsd group your interfaces on the two devices are essentially independent of each other and your devices are Active/Active for traffic.
With vsd groups your setup the failover of interfaces from one device to the other when the failover conditions are met. So your main design question is will the cluster be active/passive or active/active. Most setups are active/passive where if a failure occurs all traffic shifts to the backup passive firewall during a triggered failover.
Then proceed with the basic setup of the cluster from there.
active/passive
https://kb.juniper.net/InfoCenter/index?page=content&id=KB6015
active/active
http://kb.juniper.net/InfoCenter/index?page=content&id=KB9809