How hard is laying tile for DIY

Back splashed as easy as it get in general for tile. No waterproofing to worry about and the bond only has to be good enough to hold the tile up.

That said its still a job and depending on the type of tile chosen could be difficult to get to look good. An example is subway tile can be tricky to get straight lines all pretty. A mosaic is more forgiving.

As with any job, its very very easy if you don't GaF what it ends up looking like.

No need for thin set, use a white adhesive or mastic like Acrylpro from Home Depot.
Mark outlet location on tile and cut with an angle grinder. A simple snap cutter or slide cutter will work fine for most wall tile. Do not cover the “ears” on the receptacle but have it close enough to be covered by the plate.
Set tile level with the countertop but it’s best to raise it off the countertop about 1/8”.
As previously mentioned the ends will require some thought and careful cutting or grinding.

This is an easy DIY, just take your time. A little time and patience will have you admiring your work for years to come. If you rush it, you’ll probably screw it up and stare at your mistakes everyday until you tear it out or buy some big arse cookie jars.