At a high level, here are a couple of tips that I find helpful.

First, look at your actual transactional data to see what dimensions are important to you.  Once you determine the actual dimensions that are of value, create a forecast sheet that includes those dimensions.  Just remember, the more detailed you are with dimensions the more lines of data will be required in your forecast.  However an hour or two of pre-work when setting the forecast can allow for much more detailed tracking throughout the year.

Second, make sure the metric and dimension names in the forecast align with those in your actual data.  Without that, it is much harder to set up cards to show comparisons.

Third, we often place the forcecast data in a Domo online form, so it can be accessed and updated as needed.

Lastly, coming up with the forecast numbers is a bit of art and science.  Obviously a good place to start is with prior history.  But internal initiatives, market conditions, and other changes will affect future performance.

If you have more specific questions, feel free to post.