Unfortunately, I am no great expert and certainly not a music teacher. I tell them there is quite a lot of help available on the internet (such as Lasse Collin's site and Charlie Porter's videos) and I have referred to these in several of my articles.
These emailers tell me they hope one day to play in a band but at present they are mastering their instruments, and learning tunes and chord progressions.
Maybe you should start by watching this excellent little video, which makes very clear how the trumpet, trombone and clarinet can improvise collectively:
CLICK HERE TO VIEW.
While I was listening recently to a performance of Till We Meet Again, it occurred to me that I could at least recommend this super tune to you as something on which to practise.
Well, for a start you can take it quite slowly. Next, it includes two essential basic chord progressions that will turn up in very many tunes, so you need to feel comfortable improvising over them.
First you need to look at what goes on in this tune. So let's consider it, in the key of F.
We discover that it is a 32-bar tune (the most common type of all) and it is structured ABAB (each letter representing eight bars).
So you have two 'A' sections that are pretty much identical. These eight bars (marked in red below) use one of the most common chord progressions:
The 'B' sections use The Sunshine Chord Progression (also used in dozens of tunes). I have written about The Sunshine Progression in several articles. For example, click here to read one. Every jazzer must get the The Sunshine Progression into his fingers - in a range of keys.
In the first use of this progression, Bars 15 and 16 hold on to the dominant 7th (C7) rather than resolve completely to the tonic. The purpose of this is to lead back to the melodic theme all over again in Bar 17.
But when we reach the final eight bars of Till We Meet Again (B for the second time) we find the full Sunshine Progression - ending on the tonic to round the tune off perfectly.
So here is the full chord chart (in F):
Don't forget that if you are a Bb or Eb instrument, then the Concert key of F will become G for you (Bb instruments, i.e. most trumpets and clarinets) or D for you (Eb instruments).
To give you some idea how this improvising-on-the-chords business works, I put the tune into Band-in-the-Box and then let my computer play it while with my cornet I tried to play notes from the arpeggios of the chords. I mostly used notes above the melody, in order to avoid clashing with it. To watch my attempt - or play along yourself - CLICK HERE.