Create an Advantage to attach the situation aspect Surprised to one or more targets. The character attempting the ambush typically rolls their Rogue prof. vs. targets’ Marksman. Always use a passive value (represented by the appropriate profession), if the target(s) are not actively on the lookout for anyone. See the rules on Affecting Multiple Targets, p205 – Running The Game.
Once a conflict is established roll for initiative to determine turn order (active opposition using a relevant skill, rank PCs and NPCs accordingly, as DM you may group monsters and NPCs as appropriate to save time rolling). The Surprised aspect (or any other relevant aspect), may be invoked during this phase.
Sneak attacks can be handled by stunts, as per the Freeport Companion, e.g. Sneak Attack (Rogue): Attacker gains a +2 to Rogue-based melee attacks when his target is unaware of him.