We used a 2mm thick glazing tape in black, placed in the rebate first, then push the unit in, then Silfix U9 in black around the perimeter with the timber glazing beads squished into it and excess trimmed off. Brass pins in glazing bead (drill pilot hole first!). Top coat of paint afterwards to seal glazing bead to frame and pinheads. Black tape and silicone because we have black warm edge spacer bars.

We have grey spacers in our glazing, with white frames inside and dark brown outside. I suspect that not-shiny-aluminium is the important part

Yes - you have two lines of seal - one inboard where the DG unit's inner face is bedded or glazing-taped to the frame rebate - and a second one outboard where the glazing bead is bedded or glazing-taped to the DG unit's outer face. The DG unit's edge-face should be left bare (don't bed it 'solid'), so there's a continuous clear cavity all around the DG unit's edge. (This assumes glaze-out; same principle if glaze-in). Assume water will get past the outboard seal into that clear cavity, and make sure is can drain out at the bottom, as tony says.