Curves frequently need easing in. To begin with use the pin method . Pin in middle of each pieces separately. Pin at ends pin at quarter and eight positions on both. Now pin the together when stitch just a gentle stretch may be needed but still keep pins correct. As you become more proficient you can do the block curves without pinning. Utube have videos showing. Borders are always measure in centre of sides and pin as much as is needed. This keeps quilt square and no wavy edges.