No big deal. You use the same medium, on which you got the callus, but liquid. Divide the callus into small bits . Combine the callus and the liquid medium in an Erlenmeyer( maybe 20 ml in an 100 ml Erlenmeyer) and let shake by 100rpm.

you can use the same medium or half strength medium for initiating the suspension. Crush your callus into small bits and transfer to liquid medium and keep in a shaker. Remember to subculture at weekly intervals to fresh medium. Meanwhile,  during the subculture you try to remove the larger chunks by manual picking up or through sieving so that there remains only smaller cells in suspension. Finally, within two months you should get a fine suspension , more or less a cell suspension. Regular subculturing is a must. 

1. Carrot (Daucus carota L.) suspension culture is a classic one (for a long time, people use it for somatic embryogenesis study), and the calli are 'soft'.

2. If you don't have callus, you need to initiate an experiment to obtain callus. Don't know your purpose of doing suspension culture, but freshly induced callus is preferred (not old and repeatly subcultured callus)

3. I am attaching a paper (2005) for you about this. In the paper, it described and compared 3 methods of doing carrot suspension culture.