For french horn players, it is much better to play whole step trills using only your lips to change the notes. For half step trills, you will have to use the 2nd valve.
You can download Lip trill routine below and practice these exercises about 10 minutes/day to develop that technique. If you work on it every day, it should take you a few weeks/months to get decent lip trills.
A few tips: -Play the bottom note slightly sharper to bring it closer to the upper note. It'll make it easier to move from one note to the other. -Try to relax. It'll help you keep a good air flow. - Don't forget to make a nice sound when you trill. Tone quality always counts. Even during lip trills! -Trills in the high register are more difficult but practicing them will make you stronger and you'll find it easier to play the lower ones afterwards.
You can download the first page of the Lip trill routine for free by clicking on the "view sample" button below or download all four pages including the fingering chart four 0.99$.