Assembling completed garment

ive finished a sweater and would like a method for the flatest seam possible

Most likely mattress stitch (on vertical seams). (There’s a vid if you need it.) It may depend on what sts were used in construction. Mattress might not be viable in a lace pattern.

For horiz seams, like shoulders, there’s helpful pics/info here. Three-ndl BO is one of the best and strongest for shoulders but requires live sts (you can undo your BO and work the 3-ndl.)