The quick answer is, by connecting to +ve side of your supply to your reference point.
Consider that you have a source of voltage, maybe a battery, it is not connected to anything.
Take a meter and measure the voltage coming from this source, lets say it measures 12 volts.
One terminal of our source is labelled +ve, the other -ve.
So have we got a +12v supply or a -12v supply? It all depends, we need to define a reference point.
Say we put the source into a metal box with a view to powering some other circuit in the box.
If we connect the -ve terminal of the source to the box, and say that the box will be our reference point, inside the box we have a +12v supply.
Or we could have connected the +ve terminal of our source to the box, in this case we will have a -12v supply.
I hope this helps.
JimB