Center the steering wheel, then make the adjustment.

The steering wheel isn't going to move while you are making the adjustment, so you don't have to lock it. Rather the wheel is going to pivot in/out.

You'll need to maintain about a 1/8" toe-in between the two wheels, so both tie-rods may have to be turned.

If, for example, the steering wheel is centered but the car drives to the left, then you know you need to adjust the tie rods so that the wheels are pointed more to the right. In which case you need to rotate the left tie rod into the left tie rod end and/or the right tie rod out of the right tie rod end. While maintaining the correct toe-in.

You'll need to rig some kind of measuring device than can measure inside distances. Some kind of expanding rod.
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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.