The material

An everyday situation: when you are experimenting on the keyboard, you often come up with little ideas that you can reuse later. Maybe for a new trailer. Once a selection has been made and the ideas have been brought into a common key, they need to be musically worked out step by step and brought together into a meaningful whole. “One” possible way is shown here.

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The task is to create a first piano part from the short ideas. The piano part should then turn to an orchestral part. This can be quite puristic, i.e. Instruments are used only for sonic reasons. It is not a matter of to "occupy" all the instruments. Let's go.

Elaboration of ‘idea one’

You could maybe do it like this:
The 'idea one' has enough rhythmic and harmonic energy that it should definitely be repeated.
A literal repetition 'may' sound good, but a slight variation of the material could be much more appealing.

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The last bar of 'Idea one' is filled rhythmically and harmonically by the descending chords in the right hand. The chords are deliberately constructed in this way: they consist of notes that complement each other in fifths (a-d-e in fifths d-a-e; d-g-a in fifths g-d-a).
In the repetition of bar 1, the right hand now has a little more power. The following two bars are given a new colouration by the octavated upper voice, 3 octaves higher. And with the help of dynamics and articulation, the whole thing becomes even more lively.

Elaboration of ‘idea two’

You could do this:
The 'idea two' could be adopted exactly as it is. It might be worth refining the articulation a bit. And for the trailer, you could repeat it a second time, with the dynamics a bit louder the second time.

Elaboration of ‘idea three’

You could proceed like this:
'Idea three' could be given more space than the previous material by extending the two bars of the original to a classic 8-bar period. The melody part could be varied slightly so that it doesn't become too monotonous. The constant change between 4/4 and 3/4 time has a special charm. Then you could, for example, shift the 8-bar period harmonically: e.g. a whole tone lower from D minor to C minor. Somehow you have to get to the end: for example, by thinning out the movement, a ritardando and a discreet return to the original key. 'Idea two' is very suitable as a final idea: the first time around, from bar 9, this groove remained within a single harmony. At the end, the groove of 'idea two' could be made to oscillate between two harmonies. On the one hand, this looks like a flashback, but it is not a simple repetition. And the sketch would be finished...

A standard orchestra is available with the following instrumentation: 2 flutes, 2 oboes, 2 clarinets, 2 bassoons - 4 horns, 2 trumpets, 3 trombones, tuba, timpani, percussion, strings (abbreviated: 2, 2, 2, 2 - 4, 2, 3, 1, timp, perc, str).

As mentioned at the beginning, the instruments can be used according to purely acoustic aspects. Some (classical-romantic) books on orchestration discourage this technique, claiming that composers of the time would not have worked in this way. However, this rather effective way of working is often used in film music.

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TIP: To keep track of the orchestration, it is a good idea to integrate the piano part into the orchestral score first. You can then either make it invisible at the end or delete it completely.

In this scenario, only ‘one’ of many possible solutions is demonstrated, as there are various options for elaboration depending on taste. 
However, you will very quickly realise that it is not so easy to find instruments that come close to the character of the piano part at this fast tempo. In addition, the result should be ‘playable live’. In other words, it shouldn't just be playable on the computer, and since no piano is supposed to play in the orchestra, it's not so easy to transfer the percussive character to others. No mallet instruments were chosen for this scenario either, which might have been the best way to achieve this. So all in all: difficult conditions.

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Here are some tips for the instrumentation chosen in the following video. For example, low brass instruments are suitable for the low fifths in Idea 1. They can play the eighths succinctly in staccato and have reserves for clear crescendos. Woodwinds have been chosen for the repetition of the material, octavating upwards to create a 'brighter' character than at the beginning. The pizzicati used here in the violas and cellos have a percussive character, but are not as effective for a crescendo (be careful: they tend to sound more voluminous on the computer than live). 

Tips for woodwind instruments: Always remember that instruments have different characters in different registers. For example, flutes can sound very loud in the high register, but not in the low register. The opposite is true of oboes and bassoons. 
Tips for strings: Depending on the musical material, string instruments can be very effective melodic leaders, especially when played in octaves. Large leaps, staccato and legato playing with simultaneous changes in dynamics are no problem. They are also particularly suitable for pads, regardless of position. Sforzati as single notes, chords or in octaves are also dynamically effective. 
Percussion tips: Timpani, bass drum and snare have been chosen for this arrangement - this can be expanded to suit your taste. Very 'typical' tremolos (rolls) and accent beats are demonstrated here.

If you are looking for information on orchestration, there are many sources on the internet. Here is a link with some useful tips: www.vsl.co.at/de/academy/

FINAL TUNING:

Summary

Improvising a bit of music, putting the fragments together in a musically meaningful way, creating a first piano movement, transferring the material to an orchestra, working on dynamics, articulation and phrasing, using MIDI automation for refinement, and finally optimising the spatial sound for the new instrumentation:
It all sounds very simple at first, but in reality it is quite a lot of work. But it's worth it, as the comparison at the end clearly shows.