I usually try to model anything that's a permanent part of the character concept as a power in one form or another. I would do it like that here, too. That basically means, that any power the golem is supposed to have has to appear on the characters sheet and has to be paid by his refresh.
Let's say we want to have a pretty simple character. Just the guy who's building and controlling the golem, and the golem itself. Let's say a clay golem at the moment.
It would look something like this:
Feet in the Water (6 Refresh)
High Concept: Golem Master
Powers:
Beast Change [-1]
Human Form [+1]
- inhuman Strength [-2]
- inhuman Toughness [-2]
- Living Dead [-1]
Skills:
Summoner:
+4 Craftsmanship, Resources
+3 Lore, Scholarship
+2 Guns, Contacts
+1 Athletics, Investigation
Golem:
+4 Might, Fists
+3 Athletics, Endurance
+2 Intimidation, Alertness
+1 Survival, Conviction
So what's actually happening? I know it looks like the golem master is turning into the golem from the powers I gave him, but that doesn't have to be the case. It's just a matter of interpretation. The character concept hinges on 2 individuals, the golem and the master, so those two are always going to be in the scene, unless the master can't take the golem with him. In the scene, the master can either take his action, or he can go into "command mode" and order his golem around to do stuff. He can't have 2 actions, one for his golem and one for himself, so he won't break the action economy. Changing between the states is a supplemental action, as it would be for a shapeshifter.
Attacking the character is a bit weird, as you could, for example, attack the master while he is in golem mode, and he shouldn't be able to defend himself. But he can, by ordering his golem around. If the golem is too far away to do so, that's a great option for compels.
Living dead basically represents the fact that the golem, if it takes any damage, can't heal, it has to be repaired using craft.
If you want to be a bit more flexible, you can take modular abilities instead of fixed powers. You would then either have a number of golems prepared in your workshop, or you can have the power to transform your one golem on the fly. If you go for the workshop option, I'd probably grant you a rebate of +1 refresh, because you don't always have access to your stuff.