There is a difference between doing the flower arrangements for an event and practising the art of 生け花 which is also known as 華道. Based on the the wikipedia definition (below) I suspect that referring to making the flower arrangements for a concert/event/room/church (or even the Chelsea flower show) as 生け花 is akin to describing a visit to Buckingham Palace for tea with the Queen as 茶道 (the tea ceremony).
When arranging flowers as a decoration but not practicing 生け花 the more correct expression is probably:
テーブルに花を飾る , To adorn/decorate a table with flowers
When referring to the flower arrangements in say a church or a room the correct expression for the flower arrangements is 花の装飾
Based on the the following sentences in Space ALC:
However, his flower arranging was completely self-taught.
I wanted to do more flower arranging that required more creativity.
There are a number of ways of saying "I am now doing the flower arrangemnents for tonight." depending on the context and scale of the operation but if this is a small event (say a table at a party) then the following would suffice:
"I am now doing the flower arrangemnents for tonight."
If you are practicing Ikebana for some event (possibly a demonstration) then the following would be appropriate:
I am practicing Ikebana for tonight.
Definition in wikipedia:
More than simply putting flowers in a container, ikebana is a disciplined art form in which nature and humanity are brought together. Contrary to the idea of floral arrangement as a collection of particolored or multicolored arrangement of blooms, ikebana often emphasizes other areas of the plant, such as its stems and leaves, and draws emphasis toward shape, line, form. Though ikebana is a creative expression, it has certain rules governing its form. The artist's intention behind each arrangement is shown through a piece's color combinations, natural shapes, graceful lines, and the usually implied meaning of the arrangement.