Take up an amount of Hepes buffered medium in the tip of the transfer pipette, then make a small bubble by taking up a little air. Then take up some more medium - roughly the same volume as the air bubble, then another air bubble, same size as before. Then take up about 2-3 cm of buffered media and then a tiny air bubble once more. Take your embryos in the smallest possible volume of medium, lining them up side by side in the transfer pipette. Introduce another tiny air bubble when all the eggs are loaded.
Some people use mineral oil instead of or in conjunction with air bubbles for pressure control.
Once the pipette is loaded and the uterus positioned, move the petri dish lid supporting the mouse to the microscope and turn on the overhead light source. Once the lights and focus have been adjusted and the mouse positioned to suit yourself, grasp the top of the uterine horn at the edge and make a hole using a 25 guage hypodermic needle through into the lumen. The site of this hole should be near the last loop of the oviduct where it descends into the top of the uterine horn at the UT junction. Introducing the embryos at this site will place them at the correct position developmentally. The embryos in the transfer pipette are threaded through the hole in the uterus into the lumen and expelled.