For a horn player transposing goes hand in hand with sight reading. You should feel as comfortable reading a part in A basso or C as in F. Practice regularly. If you know your scales well, it will be a big help.
Here are some more tips:
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Find a system that works for you for each key. For A basso or As basso you could think bass clef + some sharps or flats etc.
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Think in tonality, in keys. In the horn part you will mostly see C-E-G, representing 1st,3rd and 5th degree in a key. Learn what 1st, 3rd and 5th degree when transposing in H (it will be F#-A#-C#).