You connect the speakers into something that feeds into the projector.
If you have a receiver, that allows you to switch inputs, and always feeds the output to your projector's only used input, then you connect the speakers to that receiver.
If you have several things that feed into various inputs on your projector, you connect the audio from those devices to your speakers. It can be all connected with "Y" cables, or it may need a switch box. Some devices will have RCA jacks, and others will have 3.5mm jacks, so adapters will be needed.
Some may even use HDMI sound, optical, or SPDIF connections. Those get complicated.