The basic command, from cleartool man mkbl:
cleartool mkbl -view anUCMView aBaselineName
You need a view associated to your target Stream to apply the baseline.
I recommend using the -full option, in order to create a full baseline (rather than an incremental one by default).
Using a snapshot or a dynamic view for mkbl shouldn't make a difference, but I like working in a snapshot view, and setting my baselines in a dynamic view (in which I am sure I don't have any pending checkouts).
See:
Note: this is for a full ClearCase installation, not CCRC (ClearCase Remote Client).