Calculate the base sales forecast (baseline) for 12 months.

I have a task: To calculate the base sales forecast (baseline) for 12 months. This is a task for demand planning.
I have 200+ SKUs, a table in which there is a sales history for 2024, promotions that will be held in 2025, and 2026 is empty, for which it is necessary to calculate the basic sales forecast. Among the positions there are those that have a sales history for the whole of 2024 (some moonths have zero values, some do not), and positions that have high sales in the middle of the year (2 to 5 times higher than the neighboring months), and those with a sales history that begins in the middle of 2024, and before that they were not sold.
This is the first time I've encountered such a task, so I need to understand how to do it, at least the basic algorithm, general idea. I tried cleaning the data using the median, removing zeros and spikes (if the value in the cell is greater than 1.5*median, then put the median), but I just don't understand what to do next?