Crochet: Picnic Blanket

Hi, this time I would like to make a picnic blanket one of my daughter's friend.
They, all like to go camping, on that time they can use as a blanket or picnic table cover... this will be very useful :))

I use
100 gr red balls (7 ballas + half of Red ) + 1.5 Grey
hook: 4.5 mm

I started 203 ch.
(If I say according of the pattern: 25 squares x 30 squares, then all around grey edges)
and I would like to explain this pattern with my chart.