Using computer calligraphy, you can print your address or other information in a light grey, and then trace over with a nice pen, creating a calligraphy feel.
Cut a piece of cardstock the size of the envelope or item and add bold straight lines. Place the item you wish to calligrapher on top of the template to assist in writing in a straight line.
A calligraphy marker is easier to use than a traditional calligraphy pen.
When you start addressing your envelopes, take your time per envelope. If you pace yourself, writing 10 envelopes per day, your calligraphy will look consistent and not sloppy. Sitting down to write all 90 or more envelopes will make your calligraphy handwriting strained and it will show.