Synths are the stereotypical pad element in a mix. They’re what fill up space and create the atmosphere the musicians want out of a song.
Almost always, a synth or keyboard will connect to a stereo DI box. In the extremely rare case in which you’ll need to mic a keyboard speaker (the most common would be a Leslie cabinet), use the same techniques as a stereo guitar amplifier.
In the Mix
As synthesizers are usually the pad element, they are usually found panned hard right and left in a mix and EQ’d to remove some of the much midrange. This keeps them out of the spaces the lead and foundation instruments take up, giving you a cleaner mix. If the synthesizer is operating as a different mix element, of course, it’ll need a bit different treatment, and you can use techniques similar to that used for whatever element it is emulating.