You must learn all three of them and then use the one that suits you better.
Here you have all this conversions, done using the three methods. Click on each method..
In the solved excercises from this site, I don´t tend to develop the unit conversion (so I don´t cram you with algebra), but, from time to time, some conversion may appear. I leave you those cases here, if you are interested and wanna have a look: