Don't know whether it was patience (in the beginning) or sheer stubbornness (later: I.want.to.finish.th
is!) that made me pull this one off

, but thank you for all your lovely comments! So much praise... you made my day

@Myrdda: I found it in the book '201 crochet motifs, blocks, projects and ideas' by Melody Griffiths. As it is just dc around, I hope it's ok for me to tell you. I started each round with 3 dc and completed it with a ss to the 3rd ch. This will leave you with small gaps in the middle, so perhaps it is better to do it as the book says, so as to hide those gaps in a corner.
ch 5, ss in 1st stitch to form a ring
round 1: 5 ch, (4 dc in ring, 2 ch) 3 times, 3 dc in ring, ss in 3rd ch
round 2: ss in corner space, 5 ch, 2 dc in same corner space, * 1 dc in each of next 4 dc, (2 dc, 2 ch, 2 dc) in next corner space, repeat from * two more times, 1 dc in each of next 4 stitches,1 dc in first corner space, ss in 3rd ch.
round 3: ss in corner space, 5 ch, 2 dc in same corner space, * 1 dc in each of next 8 dc, (2 dc, 2 ch, 2 dc) in next corner space, repeat from * two more times, 1 dc in each of next 8 stitches, 1 dc in first corner space, ss in 3rd ch.
And then I added a fourth round of black, leaving out the ch2 in the corners and I did (1 dc, 2 tr, 1 dc) in each corner space.
@ kneser: No, unfortunately they weren't, but that is to be expected in a huge stash

. With the exception of one or two cotton yarns, they were all acrylic however. I still cannot figure out what worsted weight etc. means, but I tried to achieve the same size square with a thinner yarn by using two strands, or crocheting tighter if I had a thick yarn. The squares turned out to be more or less the same size, but if you look closely at the photo with the squares laid out on the floor, you can see that their sizes vary slightly.