#Help them define what they want to play
#What's the concept? First and foremost, before you can help them with decision-making, you should help them understand what they're trying to achieve. At this point, they (may) have a character concept in mind. Get them to explain it to you. Work together to create a list of what they want their character to be and do. This baseline will help everyone determine a guideline for builds and help them in making decisions they may need to make later.
##Campaign levels THe next consideration is what level the players are currently at and what level they will achieve in the campaign. Understanding this will help immensely during any upcoming decision process. Creating a 'build' that comes online late or after outside of the range you're going to play will immediately become unhelpful. Helping to make sure they've got a concept that they understand is playable for a good percentage of the campaign may help increase their fun.
##Build decisions Here is where it starts to get trickier. Multiclassing is generally about making trade-offs. By opting to multiclass, you are delaying your primary class progression and potentially cutting off certain milestones depending on how deep you dip and how long the campaign will go on for. This is where your role really comes into play. Helping players guide their decisions to fulfill their concept without handicapping themselves to an extent where they aren't 'keeping up' with others is going to be a consideration.
##How do you help? This is the trickiest. Understanding the classes yourself is a good first step. But as you help guide them into the decision-tree of what they'll get, and what they'll delay and lose, may take some more work. If they're not savvy enough, then you'll need to be - or you'll have to be willing to retcon and change things if they're not working out and folks aren't having fun. Here are some considerations to keep in mind:
- What's the end-level for the campaign?
- What level range will most of the campaign take place at?
- What abilities does the player want?
- If choosing to dip, what will they lose in their primary class?
- If choosing to dip, what ability scores/feats will they miss out on?