FL SynthMaker can be used to make synth plugins, effects and dashboards (to control external gear). There's also plenty of pre-made plugins waiting to be used. This tutorial shows how to create ring modulator and echo/delay effects.
FL SynthMaker is included FREE in FL Studio Producer edition and higher
Tip: To learn SynthMaker it's a good idea to start with some simple projects that recreate something you already know well. For example, a 2 oscillator synth. What do you know about synths? So they have Oscillators - Envelopes - Filters - Output. Then all you need to do is find those modules in SynthMaker and link them together, done!
Making a basic synth will be the subject of our next tutorial.