weave 'weave name' target is the usual syntax, though some weaves take multiple targets and some take none.
The mana amount used in a weave is the amount of mana currently grasped unless this is lower than the min or higher than the max for a weave. If the auto- force flag is disabled, you won't be allowed to channel more than you can safely grasp, exhaust your mana reserve or weave something that is beyond your flow skill abilty.