I think most makers glue up the whole handle slab(handle material and liner) first then completely finish the front of the scale before glue up.
It depends on the adhesive you're using but in the past I've epoxied bolster to handle material first, flatten, then glue to liner- that way there are fewer parts that can shift. A couple of clamps to hold everything in place is all you need. I would clamp your bolster to a table, apply glue between bolster and handle material, lightly clamp the handle material to the table and then use another clamp to make sure the two pieces are drawn together. If you use cyanoacrylate to glue the handle material to the micarta bolster then hand pressure and maybe a clamp is all you need until it's set then flatten and glue to your liner. After everything is dry, drill your pin holes and install.
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