A loose arrangement should look easy, as if the flowers fell into place on their own. That ease comes from a few small decisions. None of them needs training, only a little patience and a willingness to take stems out again.
Start with height
A common starting point is to let the flowers stand about one and a half to two times the height of the vase. Treat it as a guide, not a rule. A low, wide bowl suits a short, spreading arrangement, and a tall vase suits stems that reach upward. Set your tallest stems first, then work down and out from them.
Work in odd groups
Groups of three or five usually look more natural than pairs. Place a few stems of one flower together, then repeat that flower in a smaller group elsewhere, so the eye moves around the vase without landing on a neat row.
Leave some space
Gaps are part of the design. Leave room around and between the flowers so each head can be seen, and resist filling every hole. Space is what makes an arrangement look loose and not crammed.
Let stems lean
Most stems have a natural curve. Let a few lean out over the rim instead of forcing them upright. A stem leaning to one side can be answered by a lighter one leaning the other way.
Use pale flowers to give bright ones room
Large, round, strongly coloured flowers look heavy, and fine, pale or spiky ones look light. Place the heavier flowers low and near the centre, where they steady the arrangement, and use paler flowers and foliage around them so the brighter colours have room to stand out. The aim is that neither side seems to tip.
Turn as you go
Rotate the vase every few stems and look at it from the angle people will see it from. Turn individual stems too, so each flower faces outward and shows its open face.
Edit by taking away
When something feels off, try removing a stem before adding one. An arrangement that looks busy often needs one fewer flower. Step back, look again, and stop when it feels calm.
Let it change
As the days pass, some flowers will open and others will fade. Take out tired stems, and when the vase thins out, move what remains into a smaller vase. A single stem in a bud vase is a lovely way to end.