Hi Lucy10! I run demostrations and workshops in both Watercolour and Acrylic and I love busy skies!
We are all different, but my way of approaching a sky as you describe would be firstly, ensure that you stretch your paper; there's nothing worse than try to lay a smooth wash and have a ripple leave a darker patch, probably leading to a cauliflower!
So, after I've stretched my paper, I wet the sky area down to the horizon line if its either a landscape or seascape.
Then with a fairly strong mixture of Cobalt Blue, I randomly lay in area's of the sky I want to be blue, leaving plenty of white paper.
Into the Cobalt Blue on the paper, I then lay in some areas with Burnt Umber, and let that spread into both the blue and the white area's of paper.
I sometimes use a dab of Kitchen Towel to lift out the odd cloud, but I much prefer to use the "Paint Magnet"; this is a clean brush, dipped in clean water and squeezed out between finger and thumb, and then mould in the clouds with this - it really is a paint magnet, as it sucks paint from the still wet paper, and I find it easier to shape clouds like this.
As with anything we try, have a go, then another go, and you'll get better and better!
If you want any further information, please come back to me and I'll help as much as I can.
Good luck and very best wishes,
PA & RSC Wigan area