I don't press the knit pieces but I do block them depending on the yarn used. Which yarn are you working with?
Seam together the fronts to the back at the shoulders. You can use a mattress stitch or a back stitch here. Here's a video for mattress stitch which works well:
To knit up or pick up stitches for the buttonhands along the fronts, mark off the front into halves or fourths and pick up the appropriate number in each section. That makes it easier to pick up fairly evenly along the front. Here's a video for picking up sts:
We'd love to see a photo of the finished sweater!