How is iron(III) chloride changed from solid to solution?
You can prepare a solution of $\ce{FeCl3}$ by dissolving the appropriate amount in water. 

Let's say you want a $0.1 \text{ M}$ solution of $\ce{FeCl3}$. You would do this by dissolving 0.1 mole of $\ce{FeCl3}$ in 1 liter of water. 

$$0.1000 \text{ mol} \times \dfrac{162.2\text{ g}}{1\text{ mol}}=16.22 \text{ g}$$

You would need to dissolve 16.22 g of $\ce{FeCl3}$ in enough water to make 1 liter of solution.

This solution is not stable over time. $\ce{FeCl3}$ hydrolyzes in water. See my answer to this question. The hydrolysis can be slowed/reversed by adding $\ce{HCl}$. Since solutions of $\ce{FeCl3}$ are already acidic, there is no harm usually in adding more acid. For this 0.1 molar solution, you might choose to dissolve 16.22 g of $\ce{FeCl3}$ in enough 0.1 M HCl to make one liter of solution.