Forget about printing on some transfer paper and then ironing it onto some fabric. With some freezer paper you can print right on the fabric itself. No need to reverse the image and it's faster, cheaper, and more effective.
Use pinking shears to cut the top edge of your muslin. Cut your material exactly the size you are printing - 8 1/2 x 11, 11x14, 11x17, or 13x19 -- whichever your printer takes. Iron fabric to the shiny side of freezer paper. Leave just about 1/8 inch at top where pinked edge is so the printer catches the freezer paper first. MAKE SURE there are no threads ANYWHERE to catch in the printer or you will hear loud whining, groaning, dying sounds. Set printer to cardstock or heavy paper and to the size you are printing. Put in printer so material side gets the printing. Then --- hit the print button.
To make an iron-on transfer take a sheet of regular computer paper, run a glue stick around the outside edges and lay this over a piece of freezer paper--to the non-shiny side. Trim your freezer paper to fit the computer paper. Start your iron warming on a cotton setting--no steam. If your pattern has any text on it you will need to scan it into your computer as a mirror image--because otherwise it will print backwards! After you scan and save your new image print it out--this is the copy you will print from. Place the piece of freezer paper you created into the printer so that the printer will PRINT ON THE SHINY SIDE of the paper and print. Now take this directly to your iron (I don't know if you can let it sit for a long period of time and have it still work or not) and iron it face down onto your fabric--making sure to press the entire area. When finished pressing peel back the paper and voila'! You have your image all ready to stitch! Sure beats holding it up to the window.