Decide what you want to do with your finished garage. Depending on your needs, your design may require special factors, equipment or even zoning.
Check with your local zoning office. Make sure you're allowed to effect the changes you want to make. Find out about any restrictions on use, building or materials. Do not fudge on this step--if you skip it and get caught, you'll have to tear down the changes you spent your time and money on.
Make a list of any special or unusual requirements your remodel may need. A home business may need Internet or phone lines. A shop will need heavy-duty electrical wiring, maybe even a dedicated fuse box. A rec room or bedroom suite may need a bathroom, and thus plumbing.
Based on your special needs, calculate a budget for your project. Many people begin with this step, but it's better to do the figures after you have a general idea of what you'll need.
With a tape measure, measure the dimensions of your garage. Include wall length and height and the position of any doors, garage doors and windows. Also note the location of existing electrical outlets and light fixtures.
Draw a rough sketch of your existing garage, including all of the factors listed above. Graph paper works very well for this.
Draw a rough sketch of what you would like your refinished garage to look like. Use three different colors: one for what exists and will remain, one for what exists and will be removed and one for new construction.
If you are not a professional architect or draftsman, get a professional to convert your rough sketch into formal plans. Most cities require a licensed professional to draw those plans to get your construction approved.