This page has some good advice on and descriptions of ways to attach gear on the outside of your backpack. Sectionhiker.com
It’s taken me a few years to figure this out. I’ve had a cheap old tarp in the past which I’ve almost never used; then, before my John Muir Trail hike I picked up a ultra-light Sil-Tarp (below) which was marginally effective but I still never really got the hang of pinning it down in the wind.
And then here, below, is the cheap one I got in Hanksville, because it rained almost every day in the Escalante National Monument. But these cheap tarps are also unwieldy in the wind and tend to collect rather than shield rain.
I purchased a Kelty tarp a couple of years ago; have had difficulty in anchoring it for a couple of years, but this last time in Utah I finally got it staked down well (the pic at the top of this post). In a big wind, I still had to anchor one peg with a stool and a large water bag on top of it to make sure it didn’t pull out.
Tarps can be crucial for enjoying your car camping experience; less so for backpacking I think, because when you’re backpacking, you’re going to be on the move the next day anyway. But car camping, you’re probably going to stay put for a few days; the tarp can keep you protected from intermittent showers with little discomfort; you can cook underneath them, hang out, snooze, whatever. Beats the heck out of spending sixteen hours in a tent because of rain.
ZPacks accessories sell some pretty nifty lightweight gear for hiking and backpacking.