From the Avatar Gallery, found in Basic and Advanced UI, go to Props tab and start exploring.
When in Props tab, click on Add Prop and a list with available props will open. Select the prop that you want
and also select the bone link which the prop will be pinned to.
You can also leave the bone link on default.For example if you want to add the red ball on Fluffo's nose, choose the bone link called LNose. Click on Attach to add the prop. After the red nose is attached, if you scroll down you will be able to use the sliders to adjust the prop's size, position, rotation, roll etc. Fiddle around with the options to create the effect that you want!