Transposition is not at all difficult, but if you really can't count round the letters, then you could make a transposition slide rule, as follows:

Write all possible key names, TWICE, evenly spaced on one ruler (Or piece of paper) like this

A Bb B C Db D Eb E F Gb G Ab A Bb ...etc

Write all the key names again, similarly spaced on another. Then place the two side by side, and slide until the keys line up.

Remember that only the NAME changes, not the type of chord.
For example, to transpose from C to Eb, just place the C on the one slider against the Eb on the other, and read off all the other chords.

You may also note that some keys have TWO names, sharp or flat.
The following is EXACTLY equivalent. Make another slider if you prefer, or write the sharps above their equivalent flats.

A A# B C C# D D# E F F# G G# A A# ...etc

Rule: If your key has a # in the name, use the # names, if a flat, use the b (flat)names


ALSO note the 'enharmonic equivalents' :

Cb is the same as B
Fb is the same as E

B# is the same as C
E# is the same as F
You won't usually see these though.

You could include these all on one slider if you like, as below
So to transpose from , as G to C#, you just place the slider as

        Cb B#          Fb E#              ...etc
     A#       C#    D#       F#    G#    A#
  A  Bb B  C  Db D  Eb E  F  Gb G  Ab A  Bb B  C  Db D  Eb E  F  Gb G
  -------------------------------------------------------------------
              G  Ab A  Bb B  C  Db D  Eb E  F  Gb G  Ab A  Bb ...etc
  

So G becomes C#, D7 becomes G#7, Am becomes Dm and so on.