A method and apparatus for forming a lesion in tissue along a desired ablation path with treating a body tissue in situ (e.g., an atrial tissue of a heart to treat) atrial fibrillation include a lesion formation tool including is positioned against the heart surface. The lesion formation tool includes a guide member having a tissue-opposing surface for placement against a heart surface. An ablation member is coupled to the guide member to move in a longitudinal path relative to the guide member. The guide member includes a track. A carriage is slidably received with the track. The ablation member is secured to the carriage for movement therewith. The guide member includes a visualization component.