It takes a little while to get used to driving on the opposite side of the road. For the first day or two, it will feel odd, but most people do not have much trouble adapting.
In most cases, you just have to follow what traffic is doing. Places where lanes aren’t clearly marked are the most common causes of trouble (for instance, waiting to exit a parking lot, you might find yourself waiting on the wrong side and thus blocking people trying to get in).
When starting out, watch out for the side mirrors, especially on narrower roads. Your sense of where you are in the road will be off-kilter for a while. Your first couple times wrong-side driving, it probably pays to take out a low-deductible insurance policy from the rental agency — you may well damage the car, although it will usually be a minor thing like scraping the wheels on a curb (which agencies sometimes don’t charge for), scraping the side on hedgerow, or taking off one of those wing mirrors.
And, if you have a manual-shift car, you’ll find yourself shifting into the wrong gear — not catastrophically, but you’ll often be one gear off where you intended to be.
Last point! In a few countries, like Thailand and Japan, the windshield wiper and turning signal levers are also reversed. So you’ll be using your wipers a lot by accident.
Ironically, if you are a confident driver who likes to take corners aggressively, it can take longer to adapt, because the balance of the car will feel wrong when you are going fast around corners.
Wrong-side driving in Thailand. Unlike the UK, the windshield wipers and turn signal levers are also reversed. Photo by the author.