A lot of the templates have some nicely stacked and shadowed cluster "blanks". JBStudios, CarolW designs are a couple that come to mind. You could simply choose and place your chosen flower, leaf, ribbon over those spots, add your shadows and you are done. As far as choosing elements, I rarely stray from single kit use but I do have a separate folder in which I place flowers, leaves, ribbon, lace etc when I come across something I particularly like and want to use again.
I have a cluster fetish!! It is one of the things that gives me great pleasure to create. I like big clusters with lots of ellies. My background is in cake decorating and floral arranging which has been helpful..isnt it wonderful when hobbies interact!! My process is to start with a flower or flowers, angle and size them differently if using more than one (3 is a pleasing number). If you choose to repeat the same flower, you can do level adjustments to slightly differentiate between them. That flower or flowers become the focal point or anchor and everything then angles back to that central point... for instance when you add a stem, leaf or leaves, follow the line of the foliage stem and place it so the line angles towards the centre of the focal point. C shapes and S shapes are pleasing to the eye, curved stems can help create that design. To gain a perception of depth, dont be afraid to layer foliage OVER the flowers as well as under. Uneven numbers are mostly pleasing but I dont get OCD about that, I go with what looks good to me. Shadowing is important to create depth.
This one has a strong C shape and if you can envision imaginary lines from each element you will see how they all roughly lead towards the single focal flower in the centre
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I often place the finished cluster into a Group (in photoshop) or link them all together. I can then duplicate, reduce or enlarge, or move all in one go.
I dont think Ive done a very good job at describing my method (but I know what I mean

)and it is by no means the "correct" or "only" way to create clusters, but it is my way and I hope some little thing here might help you in creating your own.