A quicker way to deal with this problem , and how I created the image below is by following the subsequent method. You will have your camera set to manual and also have your on camera speedlight set to manual. You can do this with a manual speedlite only, ETTL speedlights won't work. To begin with , you will use your light meter to obtain the exposure of the window's ambient light. You will then set your camera to that reading say f8 @ 1/200 th of a second. You must know the sync speed of your camera. Most high end DSLR cameras, like Canon's 5D Mark II, my choice, sync at 1/200th of a second or if you are using a Nikon,1/250th Thus you must keep your shutter speed under the sync speed of the camera you are using. If you don't do this, half the frame you shoot will be dark as the shutter will close much too quickly. There are ways to get around this but we won't be addressing them here.
Now that you have your camera in manual and set for your ambient light reading you frame up your interior shot without changing your aperture or your shutter speed. This will show you to be far too under exposed for the room but we will be using the speedlight to compensate for that problem. You now will alter your flash manually to a power setting that will bring up the interior light while keeping the same f stop used for ambient reading...[shutter speed not f stop controls the amount of flash]...It is best to tilt your flash towards the ceiling to help diffuse the light, tilt it forward just a click or two so it will throw the flash off the ceiling and fill the room. You usually will have to shoot a couple frames and alternate the power output of the speedlight until you get your desired effect.
Now that you have the desired exposure you are only dealing with one frame...you will not have to do any masking as in the first method though you will still have to adjust exposure in post and play with the white balance and the shadows and highlights. Also this quick fire method tends to give a rather sterile look to the room as the light is very even which is not really natural.
Despite the drawbacks, this method is far quicker as the first as you do not need to set up a tripod and balance it all out etc. You will not need to create layers in post either. Basically your work flow will be twice as fast and this is important if you want to make some money as most realators don't want you spending more than a couple hours at the very outset in the home. If it is a 5 million $, 5000 square foot home, this is next to impossible, and really deserves more work to make the best images possible. Many of these homes sell on line simply from the photos. I shot one a couple of years ago and it sold the night i posted the images to a man on the other side of the country so it really is worth the extra investment a realtor puts out, the commission will cover it anyway. Despite this, you will be surprised how many realators expect you to get it done quickly to save a couple hundred dollars. Architects and Interior designer tend not to push the clock as much as real estate agents.
Well I hope this little tip helps somewhat. There are other a number of other ways of shooting interiors, for example using constant light. Personally, I prefer the continuous light method as the illumination is much easier to shape and model . However, it can also be very time consuming as well. Remember to practice different methods often before going out on a job. On the job is not the place to try out new methods. If you do so you will probably find that you will never hear from that client again !