Building a financial forecast

I'm building a financial forecast and for the life of me cannot figure out how to get started. Here's the data model:

table_1 description
account_id
year calendar year
revenue total spend
table_2 description
account_id
subscription_id
product_id
created_date date created
closed_date
launch_date start of forecast_12_months
subsciption_type commitment or by usage
active_binary
forecast_12_months expected 12 month spend from launch date
last_12_months_spend amount spent up to closed_date

The ask is to build a predictive model for revenue. I have no clue how to get started because the forecast_12_months and last_12_months_spend start on different dates for all the subscription_ids across the span of like 3 years. It's not a full lookback period (ie, 2020-2023 as of 9/23/2024).

Any idea on how you'd start this out? The grain and horizon are up to you to choose.