In Short:
1. Use Keystone of Trials, dont do anything --> you will get a GRift Lvl 1 Keystone
2. Enter GRift Lvl 1, kill everything except the boss, wait until time is over OR nearly over if you want to upgrade your keystone (do 1 or 2 bounties, idle, whatever)
3. Re-enter GRift and kill the boss --> Profit!
4. Upgrade Keystone to lvl 2 --> repeat from step 2 OR use a new one
Have fun