Inner tube sizes need not be as precise as tyre diameters. Rubber stretches and the same tube may fit several close but different tyres. A too wide tube gets harder to fit without creases a too narrow tube will stretch and get thinner and put more stress on the valve seat. In an emergency a longer tube will fit in a small tyre and hold air for a few kilometres before it will fail because it's folded.
Inflate inner tube Gently
Inflate the inner tube until it just holds its shape.
If the bike is to be ridden in a ‘high performance’ context dusting the inner tube with talcum powder makes the tyre work better as the inner tube can move around more easily inside the tyre. It also makes it easier to fit a borderline too-wide tube without folds or getting stuck under the tyre bead.
Fit The Valve
Push the ‘off’ tyre bead, towards the ‘on’ tyre bead to expose the valve hole. Push the valve through the hole and feed that section of the inner tube into the tyre.