Achillea is a great plant to have in the garden, very robust and will grow pretty mucjh anywhere, full sun, partial shade or anything in between. The onl;y place Achillea does not  like is really wet conditions. 

Achillea will spread, sometimes quite quickly so a small clump will become large in a few years, this is a good thing as the plant can be split quite easily. We all enjoy free plants. All perennials need to be divided and split after a few years, once a plant is ciompacted the blooms will be reduced. 

In the summer the plants will sprout long stems that can reach up to 50cm in height,  with plate like flowers at the top of the stems. Commonly seen in feilds in its wildflower form of Yarrow, the version we have in outr gardesn varies on the vartiety. Red, Orange and also yellow. There are a few variets tyhat can gorw upto 2 meters tall so check the laberl before plantinmg, these would need to be at the back of any border. Once the blooms fade dead head like any other plant.

At the end of the season remove all spikes and cut down to the ground and remove any brown foliage from the plant, then leave it over the winter, there may be some brown leaves on the plant in the spring, these can be removed once the new groweth is starting to show. 

Achillea can if not controlled take over a part of your garden, just like any other plant, left it will spread. As long as you are keeping on top of it, you will have no problems.