While refinishing wooden sash windows isn't difficult, it does require a bit of work. With some preparation and patience you'll get the results you want.
Step 1: Test for Lead Content
The first step and possibly the most important step for safety is to test the paint or varnish you are going to remove with a lead test kit. These are available at any home improvement store or local hardware store. It is especially important if you live in an older home that was painted when lead was still being added to the paint. If you find you have lead paint, you will need to consult government regulations for its removal. Chances are it will not be a do it yourself job at that point.
Step 2: Apply Stripper
Cover your floor beneath the window with drop cloths. Falling paint and varnish can dry on any surface it contacts and can be very difficult to remove and can cause permanent staining. If you are using paint and varnish stripper, it is easier to start at the top of the window and work downward. Be sure to wear your gloves during this step and all other steps. Also be sure to work in a well ventilated area and open windows if at all possible and use a fan. The fumes from some of these products can make you very sick if inhaled for a long period of time. Also do not use around fire or flames as they are flammable. Apply a liberal amount of the stripper using manufacturer's instructions on the packaging. On the average size window, coat about half of the wood at one time as you don't want to loosen more than you can remove before it dries. You may find if you have a heavy build up that you may need to apply several coats before you can see the wood beneath the paint. Don't scrape layers off between coats, just add more. Also, avoid being impatient and let the stripping agent do its work.
Step 3: Remove Paint
Once all the paint has bubbled up and you can see the wood beneath it, use your scraper or putty knife to scrape all the loosened paint. Use an old coffee can to scrape the stripped paint into from your scraper. Continue this process until the entire window casing is stripped. If you find paint sticking in corners and crevices, use a small sharp tool such as an awl to pick it out.
Step 4: Prep the Wood
Once all of the paint has been removed, pour mineral spirits into a disposable container, dip the steel wool into it and start sanding the wood to remove extra paint, varnish and stripper. Clean your steel wool often and use new mineral spirits when they become dirty. This will clean the wood and ready it for painting. Allow the wood to 'rest' for at least 24 hours before staining or painting.