I'm not sure what yoiu mean by "
the polka dots don't connect like in the completed picture."
It seems to me part of the problem with the way this pattern is written is that the cream triangles will have all BIAS edges which, by their very nature, will stretch.
It's so much less frustrating for a beginner to make triangles which will have the long edges cut on the straight grain of the fabric and the shorter edges on the bias. Done this way, the bias edges will be stabilized by sewing them to the straight blocks next to them and your outer edges of this section will be stable because they are on a straight grain.
So, to overcome the bias on the outside, cut
12.5 inch cream squares -- cut these on both diagonals to create 4 triangles each -- be sure to cut enough 12.5" squares to make as many triangles as you will need .
With this mdethod, the resulting triangles will be a bit larger than you actually need, which will allow you a bit of extra fabric to trim the edges to an even line AFTER you have sewn all of the sections together.
Just a little tip from an old quilting teacher. PM me if you wish.
Jan in VA