I'm the odd man out here. I press most of my seams open. When I pin two blocks/rows together here is what I do. Place a pin straight up/down through the seam that you are matching about 1/4" in from the edge. Closely (doesn't have to be exact) line up your underneath block/row and place that same pin straight up/down through the seam of that block/row about 1/4" from the edge. Holding that up/down pin as straight as possible, put pins horizontally through both layers of fabric as close to either side of that up/down pin as possible. Repeat this process for each seam to be matched throughout your block/row.
If you end up with some bagging/flopping fabric, put the baggier side on the bottom when you sew and let the feed dogs help ease that excess. If it's a lot of excess, tug lightly on the seam as you are sewing on it and it will stretch the top fabric to help ease that excess as well.
I was never overly successful pressing to one side and getting my seams to 'nest' the way lots of folks are able. This is why I started pressing my seams open. And when you are dealing with blocks with lots of seams, I think the blocks lay flatter when seams are pressed open.
Take your time, take a few deep breaths and you'll be fine. Remember, this isn't a race. We're supposed to be enjoying the process.