6 marks are for AO1 (definition and description) and 10 marks are for AO3 (discussion - evaluation), and there should be a clear difference between the two because the last thing you want is for the examiner to get confused and mark you down.
I won't write the whole thing, but an example structure for "Discuss biological explanations for OCD" would be:
AO1: One biological approach to explaining OCD is genetic explanation, this suggests certain genes can make people vulnerable to the development of OCD, these are called candidate genes. Research suggests that OCD is polygenic, meaning that more than one gene is involved in this vulnerability.
AO3: A strength of the genetic explanation is that it can explain the different types of OCD. It suggests that different candidate genes are responsible in the vulnerability, and that different variations can account for the different types. Taylor (2013)'s research [go into detail] found 230 variations. This explanatory power is a strength of the explanation because it increases its validity.
A limitation of the explanation is that it is deterministic, as it does not account for the environmental risk factors that are involved in the development of OCD. Cromer et al (2007) found that over 50% of OCD patients in their study had had a traumatic event occur in their life, they also found that the level of severity influenced how sever their OCD was. This is a limitation because it cannot account for other influences in its development, and is therefore not a full explanation for the development of OCD.
AO1: Another biological explanation of OCD is neural correlates. This suggests that abnormal brain functioning, such as brain structure and neurotransmitters, is associated with the development of OCD. [Explain role of serotonin and decision making systems].
AO3: A strength of neural explanations is supporting evidence. [Researchers, what they did (if stated) and what they found]. [More if any]. Supporting evidence is a strength because it increases the validity of the explanation. [But make that longer].
A limitation of [topic] is [_____]. [Elaborate on why, include research]. This is a limitation because [say why]. <--- That's the basic structure, just need to fill it out.
(I'm a bit rusty because it's been five months, but the evaluation structure is very systematic, just make sure the elaboration of the point in the first sentence is length but coherent)
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I'd recommend planning them with brief structured spider diagrams, I can't add photos so I can't show them but basically, have the question in the middle (since it's on the paper and you have limited time maybe just draw a bubble with nothing) and to the left have brief points of AO1 and on the right have four brief evaluation points (very brief, so you have something to expand on when writing it out - names of studies/researchers, brief methodological issue, basis of conflicting research etc). Planning is super important, otherwise it's so easy to just go off topic, and examiners don't have the time and they don't care enough to sit and try to decipher what you mean.
It's quite late at night, so I hope that made sense.