Eventually, this will become second-nature to you. However, until then you need to train yourself to first plan the route your conversion will take.
For example, I want to convert 1.25 quarts of water to milliliters.
You kind of have to start with what you are given. In this case, quarts. My plan would look something like this:
Each of the arrows in the plan is a conversion factor.
The BASE of the arrow is the factor on the BOTTOM. The TIP of the arrow is the factor on the TOP. So for the conversion factor to convert quarts to liters, the factor would look like this:
And there are, of course, exactly 1000 ml in 1 liter
So the conversion is:
1.25 quarts * 1.00 liters
= 1182.94 ml = 1180 ml.
You can apply this technique to any conversion factor problem.
Now go ahead and try a couple!