Get the final, FINAL, content before you do anything. Don't start worrying about converting your book into an eBook until your book is actually finished. If you're converting the eBook for a client, wait until their book is done. Then, follow this checklist.
Proofread the document. Check the spelling. Check the grammar. Have someone else proofread it and copy edit it. Cheap electronic publishing is not an excuse for poor quality, and a bad book reflects poorly for all involved.
Put the original images in a folder. Collect all of the images. You might need to re-size images or tweak them. Make sure you have the original images and not just images inside of a word processing document.
Get the text. De-Word your text. That is, Microsoft Word puts a lot of unnecessary code in the text. Save the file out as a text file and it all goes away. You'll have to reformat it, but if you want a nice looking eBook in the end, you need to reformat it. You might be okay in Word if you select all and hit clear formatting. If the file was written correctly with styles, you might be able to avoid some re-formatting.
Give yourself plenty of time to do the work. You'll need to reformat. You'll need to choose a format. You'll need to test the document on a few devices. You'll need to render the eBook a few times to fix mistakes.
Download the before-formatting checklist (pdf).