Easiest way I can think of explaining it is to do the following:
If it's to 1dp, then 0.1 / 2 = 0.05, so add 0.05 for UB and minus 0.05 for LB.
If it's to 2dp then 0.01 / 2 = 0.005, so add 0.005 for UB and minus 0.005 for LB
If it's to the nearest whole number, then 1 / 2 = 0.5, so add 0.5 for UB and minus 0.5 for LB
If it's to the nearest 5, then 5 / 2 = 2.5, so add 2.5 for UB and minus 2.5 for LB
If it's to the nearest 10, the 10 / 2 = 5, so add 5 for UB and minus 5 for LB
If you are given SF, see what column the SF you have rounded to are in and do the same to that columns value, e.g.234000 rounded to 3sf. The 3rd SF is located in the Thousands column so, 1000 / 2 = 500, so add 500 to give UB and minus 500 to give LB
UB = 234500 and LB = 233500
Use this method and you can't go wrong!