Make sure the jig is snug. If not it'll roll and you'll cut a crooked slot or more likely, never get it back exactly and have to slightly reposition the final resting spot for the front sight.
Cut in the 45 degree angles first...fine tooth blade and don't expect to keep that saw blade more than several site slots. The jig, like a file trim die, is so much harder than files and saw blades that when the blade bottoms in the cut, it immediatly starts to knock the edge off the teeth of saw and file. (Read that, dispose of blades often, use yer old, old files only for this job....no sense ruining expensive files or worse, expensive parts with dull blades and files later.)
Angle cuts completed with the hacksaw, front and rear.
After cutting in the angles, clean out most of the remaining metal with progressive side by side cuts of the saw. You can't go too deep, the jig establishes the final depth. Course, you have to know the thickness of the barrel an leave enough metal in the roof of the barrel....I like at least 80 thousands or more on the muzzle...but I've seen plenty of guns from the factory with as little as 25 thousands in the roof and they lasted for years and years and years.
Clean up the area between the angles with progressively finer files.
Do a good job working in the angles with a small fine cut triangular file. It cleans up the bottom too. You'll know when you're done, every stroke sings and the file no longer "grabbs" at the softer metal of the barrel.
Don't pull the jig unless you are sure the dovetail slot is done. Once pulled, its near impossible to get it back on right for a cleaner cut and cleaner cuts without the jig in place go from wrong to darn ugly really fast.