Sgian Dubh
Bill, as others have alluded to, I think that big lump of a chisel is too big for those small mortices you're cutting. For something as small as those mortices I'd be more than happy to put the piece of wood in the vise, or clamp it down on the top of the bench perhaps with a holdfast or an F/G/C cramp, and putter away at it with even a bevel edged chisel. Having said that, a firmer chisel would be better, and better again might be something like a sash mortice chisel. And, yes I know, some people may suck hard on their cheeks with the idea of abusing a vise by using it to hold wood whilst morticing, but I admit I've never been a purist woodworker.

I think that big pig sticker thing is over egging the pudding-- I've always thought they're better suited to digging the garden in the autumn than being used for chopping mortises in lighter examples of furniture, ie the sort of chests and things I'm aware that you build. I think those pig sticker things are useful if you're doing large stuff, like traditional green architectural woodwork, barns, church work, etc when big pieces of green oak and so on make up the framework. (Having said that, Mafell chain mortisers are even more efficient for knocking out 5"- 6" deep mortices, but that's for another forum, ha, ha.)

As to guides, eg set squares and bits of wood set up perpendicular to the edge being morticed, I've never found them much use really as you end up trying to watch too many 'markers' and can't really use any of them effectively. I just watch the long edges of the chisel and see how they relate to the edge of the wood being morticed. It seems to work well enough for me, but may not suit everyone.

I suppose there's one technique I use for morticing that may be useful to you. I start the job by holding the flat side of the blade near the tip with my thumb-- 1/2" - 3/4" up from the tip. The index and middle finger of the same hand go around to the back side-- the ground and sharpened bevelled side. This way the flat side of the blade faces me. I rest the middle finger of the chisel holding hand on the wood as a means of pivoting the blade tip to the precise position where it's needed and then whack the chisel handle with the mallet. I just keep sliding my middle finger along the wood, in the same direction as the long orientation of the mortice, and pivoting the chisel tip in and out and whacking until I've created a suitable hole. Once I've got a decent sized mortice started I can change my grip to grasping the handle of the chisel and using the existing hole to guide it and belt away and lift waste out as I go. I don't know if the technique I've described will be useful to you or not, but it might. Slainte.