Spring is the time to repot the majority of orchids. Earlier in the year I recommended that you preorder all necessary supplies so that when you were ready for repotting you would have everything that you needed already on hand. Try to set up a space to do the work where you can stand easily and be comfortable. Don't try to repot too many orchids at one time or it will become work rather than a time to study your plants and get them ready for a new growing season. Decide how many plants you want to do in a session and then give yourself time to do it without rushing.
The day before you are to repot, put the correct amount of media in a container and soak it overnight. I recommend that you take the pots you are going to use and measure out enough dry media to fill each one of them. You might get a bit more media than you need this way, but it is better to do this than to come up short and try to use dry media. Anything left over can be dried out and put back with your store of extra potting materials. You should also soak the new pots if they are terra cotta.
The day you do the potting, pick out the plants you want to work on. If you have older plants in terra cotta pots it is easier to soak the pot in a large container of water in order to soften the roots which will be growing on the pot. You can add a bit of dish soap to this soaking water to kill any insects which might be in the old pot too. Soaking is not necessary if you are growing in plastic. Many growers recommend that you unpot over a piece of newspaper and then throw away the old potting media along with the paper after you have unpotted them.
Once you have removed the plant from its old pot, remove all remaining media from the roots by hand. Work any pieces of media off carefully so as not to damage the roots. Cut off dead roots which will be mushy and grayish looking and then cut back the healthy roots about one quarter of the way to encourage new growth. I spray the plant and roots with an antifungal and antibacterial product and let it dry while I'm getting the new pot ready for the plant.
After repotting, place your plant is a shadier than normal spot for about a month and then replace where it was growing. Repotting your plants on a regular basis is very important for orchids so that their roots do not rot from media that has broken down.