When I do my HST ~ 1) enlarge square; example- pattern calls for 2 7/8", I cut 3". 2) draw diagonal line and sew scant 1/4" seams. 3) cut on diagonal line 4) press and trim to proper size.
Now sometimes because of the pattern I have to sew those blessed triangle. Then I use a leader (scrap fabric) before sewing triangle, somehow this helps.
Also what I have learned ~ start slow. Plus if you have a straight stitch plate, helps also.
Good luck