Trombone to Alto is easier. All you do is movce the key signature three sharper. (C-A, Eb-C, F-D, etc.) and read as if it were written in the treble clef (also, play it one octave above what it reads). Tenor to Alto is trickier; sharp the key sig. one sharp (C-G, Bb-F, etc.) and reada fourth lower.
Hope this helps.
P.S. if you have played clarinet or bassoon ...
Play the Tenor music as if you were using the bottom octave clarinet fingerings in repeating octave cycles (bassoon fingerings - kind of).