I need help with my forest plot


I hope someone is able to help me.
For a literature review I have to run a meta-analysis.
I have an experimental group and a control group, often with pre- and post-data but sometimes there are more data points.
For the meta-analysis I downloaded RevMan; I have to fill in the mean and SD for both the experimental group and control group. For the mean I need the mean difference which is pretty easy but I experience difficulty with finding the SD that I have to use (average population SD?).

For example:
T1 (pre) = 16.4 (mean) and 3.5 (SD)
T2 (post) = 8.8 (mean) and 3.2 (SD)
> in an example forest plot that contains the articles from which these outcome measures derive, has: mean = -7.6 (mean) and 1.27 (SD)
The study population consists of 24 individuals, 14 experimental (data above) and 10 control subjects

Does anyone have an idea?

A million thanks!