How to build a 75 ohm to 50 ohm converter
Use a transformer.  At frequencies where 50 versus 75 Ohm impedance matters, it can be quite small.  A few turns for each side around a ferrite is probably good enough.  The impedance ratio is the square of the turns ratio.  One problem with this method is that there will be some loss, and you have to be careful your core (ferrite usually) works at the intended frequency.  There are quite a few flavors of "ferrite".

Look up something called a Smith Chart.  It is a graphical means to give you values of reactive components to add to essentially change impedance.  It's not intended for exactly this application, but it might give you some guidance.  The theory and proper usage of a Smith Chart could fill a whole book, so I'll leave you mostly with the search term.

The basic concept is that a network (usually just two or three) inductors and capacitors between two signals can change the apparent impedance of one signal as seen by the other.  The advantage is that this method can be low loss, usually less loss than a transformer.  The Smith chart is a graphical way to arrive at the component value, which can be quite complex mathematically.  This is usually used at the feed point of a antenna so that the antenna looks just resistive and with the desired resistance.  The downside is that this only works at a specific frequency.  If your impedance matching has to work over a reasonable frequency range, then adding some capacitors and inductors won't work, and the Smith Chart method of determining them is unapplicable.