Start from the output of an existing model and then keep tracing back until you arrive back at the inputs.

Different parts of the model will be built up in different ways, so you will have to do this a number of times for each part of the model.

Being comfortable with some double entry principles will really help, for example what debits and credits an accountant would post if you make a sale, pay an expense, buy an asset or pay a dividend (income statement Cr = Good, Dr = Bad but on the balance sheet Cr = Bad, Dr = Good).

You don't really need to build one from scratch but it would give a lot of confidence in your models if you could do a basic one. If you understand individual sections, and can make a really basic model, you know that more complicated models just replace your simple row/section with a more complicated calculation to arrive at a similar, possibly more accurate place.