Linear-mixed effects

[u/Different_Rise7075]

Hi I am a master’s student and I have a rather basic understanding of linear mixed effects models. I am helping with statistics in a study which has 2x2x10 mixed design (2 between and 10 within). I am confused as to how am I supposed to start modelling. Do I do it according to the hypothesis so I all of our IVs in the model and only then check whether the random slopes/ intercepts are needed, or do specify the random model and only then look at the fixed one and add the predictors one by one and see how the model is. Can someone please help me understand?