First off, you should make sure to have at least 6 points for both the description and evaluation; equalling to 12 marks. It gives a nice, equal balance between the amount of description & evaluation points; one does not outweigh the other. This is the way I've been told to tackle this type of question.
The points you make should give an overview on what the theory is about. Providing key aspects of the theory should be your priority.
For example, for the Interference Theory of Forgetting you could include:
1. Retroactive Interference
2. Proactive Interference
3. Interference in the STM (Short Term Memory)
4. Interference in the LTM (Long Term Memory)
5. Similar Memories
6. Disturbance or Interference of memories
With both description and evaluation, its important that you are able to expand on your points (or in the case of the example, should be able to expand on the brief bullet points), such as (these aren't as expanded as they could be, but hopefully this is enough to help your understanding):
1. Retroactive interference is when new information interferes with old information e.g. AS spanish prevents you from being able to recall year 7 French.
2. Proactive Interference is when old information interferes with new information e.g. year 7 spanish prevents you from recalling AS Spanish.
3. Interference in the STM can occur in the form of distractions; we do not have the opportunity to process the information properly in the first place e.g. the sound of a loud drill outside a classroom.
4. Intereference is likely to occur in the Long Term Memory due to the confusion between the old and new memories which have been stored.
5. It is suggested that memories will compete and interfere with one another; this will occur more frequently with memories that are similar to each other.
6. Memories can be disrupted or interfered by what we have previously learned or what we will learn in the future.
As with any type of evaluation, the strengths and weaknesses of the theory should be noted and taken into consideration. However, the evaluation of a theory is relatively different from a study. Aspects you may want to touch upon in your evaluation includes:
1. Studies to support the theory
2. Studies to critcise the theory
3. Things that the theory explains well
4. Things that the theory fails to explain
5. Alternative Theories
6. Application of the theory
Example point for the Interference Theory:
An alternative theory is the Cue-Dependent Theory of Forgetting, which suggests that failure to recall memories is due to the accessibility problem; not that the original memories have been lost due to interference/disturbance. The correct cues need to be available in order to recall memories.
Exactly like the description, you should ensure that your are managing to expand on your points.
Of course, there are other attributes that you can take into consideration when it comes to evaluating theories (e.g. particular evaluation points for a supporting study); though I feel that the points I've listed may help you out a bit more ~
At the end of the day, just try to make sure that you know the 'ins and outs' of the theories when it comes to answering a question regarding it. It might an idea to practice some exam questions for describing and evaluating theories, as with the practice, you'll manage to get the hang of it.
I hope that this helped you out