You must determine the layout of the slides before you can effectively tune the Horn. There is at least one slide that will influence every note on the instrument: this is the main tuning slide. On a single Horn, the only slide not attached to a valve is the main slide. The main slide on a double Horn is usually located between the mouthpiece and the thumb valve (if you trace the path of the air as it passes through the Horn). Most double Horns have one or two slides that affect every note on the F side of the Horn. You can identify these slides by removing them one at a time and blowing air through the open F side of the Horn. If air escapes from the slide tubes, the slide affects the F side of the Horn. Some double Horns have a slide that tunes the B-flat side of the Horn only. Holding the thumb key depressed while blowing through the Horn (with the slide removed) will allow air to escape through the slide tubes. The remaining slides should be attached to the three main valves. The longer slides on a double Horn are for the F Horn while the shorter slides are for the B-flat Horn.
The first note you should tune is third space C. Pull out the B-flat tuning slide (if you have one) about a quarter of an inch. With your thumb key depressed, adjust the main slide to agree with your tuner. Now play middle C. Adjust the F Horn slide on the double Horn to tune this note. Now you are ready to tune the valve slides.
The same process (tuning octaves on the B-flat and F Horns) works well with the valve slides. Tune the second valve slides to B-natural and the first slides to B-flat, then adjust the third slides while playing octave A-flats. This will put the Horn reasonably well in tune with itself. However, combining any of these valves that were just tuned with other valves will result in a sharp note. If you need to use a significant number of these sharp combinations, you may want to pull one of the valve slides to compensate for the problem.
Further adjustment of the intonation will be necessary as the instrument is played. Tuners are set to the tempered scale, wind instruments are not. You may need to pull out a slide so the third degree of the scale will sound low enough, etc.