Having swapped my HDD for SSD in all my MacBook Pro's i can quite safely say it's very easy.
Just make sure you have the correct screwdrivers etc to do it.
It's a case of unscrew the back and remove, unscrew the screws holding in the old drive and unplug it, remove, plug put new SSD in it's place. Re screw it in place, re screw the back on. Switch on, format in utilities and then either reinstall OS X fresh or from a TM back up.
I currently have 2 SSD's in mine. A 500gb Samsung 840 Evo and in the Optibay i have a 240gb OCZ Vertex 3 which used to be my main drive until i got the samsung.
When running Mavericks from the laptop being completely off it took 14 secs from press of the power button to everything up and running. It's a bit slower at the moment due to running Yosemite but thats because its still in DP.
Once up and running though everything is lightning quick. Apps in the dock single bounce then open. Aperture with over 8k photo's opens up in 2 secs and all pics are visible within 3 secs after that.
It seriously is THE best thing you can do to a laptop. It's the Remapping of the IT world
This is a good guide to follow. No requirement to unplug the battery. I've never done it and never had an issue. But it's your call as it's your laptop
https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Unibody+Early+2011+Hard+Drive+Replacement/5119