Trying to knit again after a very long period of inactivity (last time, I was ten, I guess).
I am grateful to Drops for their wonderful tutorial videos and to my clover crochet when I have a problem ;)

For the three button holes, I used the one-row button hole method described in this video. I bought my buttons in this etsy shop

1 skein: 32 cm of the back (28/08)
2nd skein: end of the back (62 cm) + 13 cm of left front (8/09)
3rd skein: end of the left front (11/09) + 20 cm of the right front (17/09)
Half of the 4th skein: end of the right front part
5th skein + 5g: left sleeve
6th skein + 5g: right sleeve (14/10)

Instead of sewing the shoulders together, I chose to use the grafting technique or the kitchener stitch (or another video but in Canadian French). I used the mattress stitch to sew the sleeves and the sides together (27/10).
I also chose to reverse single crochet (crab stitch) around the garter stitches as I found that the edge was not neat (crochet 3.5) (28/10).

Conclusion
I am happy with the result: I like the color, the cardigan is warm and not too bad looking for a first real knitting project ;), just maybe a bit too big.
There are some mistakes on the back as some stitches were twisted as I was not careful when I put back the stitches on after unraveling, but it is not too visible.

If I had to knit again this pattern, I would used the moss stitch or the rice stitch instead of the garter stitch in the bottom of the cardigan and on the sides.
I would also try with circular needles, as I do not like sewing and counting the ranks.

I have learned a lot. I just need to practice again and again on how to fix a dropped stitch in garter stitch knitting, with this video.