Pretty straight forward, as alder doesn't require any filling...
If you're in the U.S., most Ace/True Value hardware stores carry Deft lacquer in gloss and satin.
You could shoot 2-4 coats of sanding sealer, and then level sand with #400 and start the satin lacquer coats, or could could just use the stain lacquer as sealer as well; CB has recommended that a couple of times, I've never tried it, but suspect you'd need to do some extra coats to get the sealing step done as the sanding sealer generally spray on thicker.
A lot of people get confused about the role of sanding sealer; it just really fills the purpose as primer for clear lacquer finishes.
Once you've applied the number of coats you're satisfied with, then it's just a matter of letting it cure out and doing final sanding and polishing.
Pic below is the alder back of one of my builds with a Warmoth clear gloss finish; should give you some idea of the final look, save for the gloss...
