Knowing the RA/DEC of a star how do I locate a star from ground? Standing in Melbourne at 4 am in the morning on 14th April 2019,  Sirus RA = 06h 45m 08.9s, Dec = -16° 42' 58"..How do I Locate this. Standing facing east I turn myself towards south approx 90 Deg and then look up about 16 Deg, I do not find any star.
Calculating local sidereal time exactly is not trivial as shown by the calculation for local sidereal time given at Approximate Sidereal Time:


  The Greenwich mean sidereal time in hours is: $$GMST = 6.697374558 + 0.06570982441908 D_0 + 1.00273790935 H + 0.000026 T^2$$ 
  where $T = D/36525$ is the number of centuries since the year 2000.


Fortunately thereare a number of online calculators that will do that for you.

You can, however ballpark it knowing that at the equinox in March (~21st, depending on the year, because you also need to take leap years into account), local sidereal time is 12 hours different than local time (without daylight saving). i.e. 6 pm (18:00) is 06:00 LST. This means that RA 06h00m is overhead at 18:00 local time.

Then add 1 hour for each $\frac{1}{2}$ month or 2 hours for each month since then. So, mid-April is about 1 month after the March equinox which means that RA 08h00m is overhead at 18:00 local time, and mid-May will be 2 months so RA 10h00m is overhead at 18:00.

From there, it is just like a clock - midnight is 6 hours later than 18:00 so you add 6 hours to the local sidereal time calculated earlier meaning that in mid-April RA 14h00m is overhead at midnight.

What this tells us is that around the March equinox (~21st), Sirius (RA 06h45m) is overhead at about 18:45, which we can round off to around 19:00 (7 pm) given the roughness of the calculation so far. We can guess then that it will set then around 6 hours later, at about 1 am.

In mid-April, RA 21h00m will be overhead at around 19:00 (7 pm), which means Sirius will have been overhead about 2 hours earlier - around 17:00 (5 pm), which means it will set around 11 pm, and in mid-May will set around 9 pm.

Melbourne is located at ~145° E, ~38° S. From this you can work out the relative angle from the declination (Sirius dec. is -16°43'). Melbourne is at 38° south, so declination -38 is directly overhead. Sirius is at dec. -16°43' (so about -17°) so it will be about 20° north of directly overhead. 

So, you would face towards north and look upwards almost to directly overhead. Knowing the LST, you know the RA that is directly overhead and can count towards the west (or east, depending on the month), and knowing that in mid-April Sirius was overhead at around 5 pm and will set around 11 pm, you can find it.

NB. James K's answer is really worth reading.