Ensure you have the right to produce and sell a digital copy of your previously published novel. Unless you've sold all rights (not recommended), you have the right to produce and market your book. However, you should look over the contract with the original publisher to make sure you haven't given them exclusive rights for a specified period of time.
Choose a word processor with PDF (portable document file) capabilities. Microsoft Word 2007 has PDF capability. So does OpenOffice, which is a free word processor comparable to Word. PDF is one of the most widely distributed formats for ebooks, since the file format is cross platform. PDF files are ideal for creating ebooks because they maintain formatting, including font, page setup and graphics, regardless of which device the end user views them on.
Open your manuscript in your word processor and format it. Since this is an ebook, you want to ensure that you format the manuscript as you want the finished novel to appear. The manuscript should be print ready, in other words, with print ready meaning "saved as a PDF ebook." Include a cover, copyright page, dedication page, introduction and table of contents.
Consider marketing your ebook novel on a website specializing in this type of material. Fictionwise is one of the most popular. You can also upload your previously published novel to Amazon's Kindle and take advantage of that market simultaneously. The combination of the two markets can bring in a respectable royalty check on a monthly basis and help extend the sales potential of work you've already published.