You can probably find a stencil online, or at Michael's or Hobby Lobby.

That looks actually quite difficult to DIY, mostly because of the curves. You can't use the straight edges of painter's tape as you would with, say, chevron stripes. You might be able to find a stencil, but, again, you'd be stuck hand-painting the curves. When I saw the pic, I actually thought it was wallpaper.

There is no easy way to paint this. Especially if you are a novice at such things. Buy some wallpaper and do an accent wall!

You'll need a stencil. You might be able to find one at royal design studio or another online shop.

This is a stencil, which can be found here:

http://www.etsy.com/listing/65963564/allover-stencil-mermaid-reusable

The stencil is also here. This website has a variety of options and tools that are helpful.
http://www.cuttingedgestencils.com/pattern-stencil-walls.html?category_id=12

Unfortunately, I would recommend wallpaper as well.

I DIY-ed chalk stripes (they were meant to be pinstripes, but now I just call them chalk stripes) on a small bathroom wall and it was a bit of a bear -- and that required only strips of painters tape.

Part of the difficulty of DIY-ing a large area with a small pattern is keeping the pattern aligned. In my case, it didn't help that the corners of my wall didn't meet at perfect right angles, either.

Here's why you should DIY it -- because there IS a stencil to do it, and because it you're very careful -- DO BE VERY CAREFUL -- and measure, measure, measure. For instance be careful to center the pattern and work to the edges, and use a level and some painters tape to keep your lines straight, etc.

But truly... a stencil is a great way to have complete control over your colors! If you did wallpaper, you would be at the mercy of the colors they make it in. By the way... if you want to experiment, you could do it on the wall, itself, OR... do it on a sheet of poster paper first.

You can make your own arc stencil (one instruction site: http://www.instructables.com/id/Make-your-own-stencil/) using either a compass or different sized pie tins as your guide. Make sure you put guidelines on your stencil so that each row lines up properly. Start at the bottom of the wall and work your way up. Hardest part will be letting each row dry before you do the next. Unless you can work some magic with guidelines on your walls so that you can do at least every other row in one painting session.

This website has all-over wall stencils too:
http://www.royaldesignstudio.com/

you can get 10% off stencils with code YHL10

I was about to also recommend finding a stencil, but unless you find a very large one that gives you a lot of the arches -- I'm talking about something the size of an open newspaper -- you are going to have problems making sure everything is perfectly straight. Check for wallpaper instead. Whether you're doing the entire room or just an accent wall, you'll still be doing it yourself, saving yourself labor costs but still getting that "I did it!" rush.

A tip that's saved my design life. When trying to paint a straight edge using painters tape 99% of the time the paint bleeds through, right... we've all been there. If you take a caulk gun and run a thin line of caulk along the edge of the tape, then smooth your finger along the line smearing the glue both on the wall and edge of the tape. This creates a bond between the tape and the wall and acts as a seal. You'll be thrilled when you pull back the tape and have a clean crisp paint line! :)

If a stencil is too tricky, have you considered making a stamp of the shape between the lines? You could do a little measuring and approximate that look with a little less effort.

http://www.etsy.com/listing/66161469/allover-stencil-scallop-reusable