Behavioural activation is something that every parent knows about and basically means gently encouraging a young person to increase physical and general activity. This technique is Cognitive Behavioural and is consistent with the idea that mood and feeling is influenced by both thinking and behaviours. In general, low activity levels are likely to maintain low mood states and the so increasing activity is generally beneficial for mood.
There is nothing complicated about this approach. It all depends on the usual things of timing and sensitivity.

First step is to try to get a general idea about the young person's daily routine. Its important to be completely non-judgemental about this and just convey interest in the young person's daily life. But it may require some gentle persistence to really get an idea about a young person's general routine such as when he/she goes to bed, gets up goes out, eats etc.

Having mapped things out, it may then be helpful to try to link weekly routines with mood.. i.e. at what times in the week does the young person feel more depressed or low etc. The aim would be to try to link low activity with low mood.

Make a number of suggestions about very small changes in routine and see which of those would be more acceptable to the young person. Never go for big changes however enthusiastic the young person becomes and its best not to aim for 100% of days. For example, if the young person decides to try to get up before midday during the week, only encourage him/her to do this five days out of seven. Its always important to leave a space for old habits otherwise the old habits will fight back too hard.

Having agreed a plan for the following week, stay interested when you next meet even if nothing has changed. Rarely do things move straight away. The important thing is to try to keep a balance between not being too casual about the plan and not being at all critical at things not progressing quickly. This response is a key part of the approach. Assume that persistence is part of the approach.

Changing daily patterns is hard for young people to do on their own. Looking for others to help with such changes is also important. Parents or other carers can easily fall back into 'nagging' interactions that may not be helpful so this needs to be monitored. The important thing is for others to celebrate positive changes and not focus on negative behaviours. Others who may be helpful are youth and "Connexions" workers who are often very skilled in encouraging positive daytime activity. See Social-Ecological Work