What you need

Steps

Open the aircraft and tap Calculate CG. Or open Bench and tap CG calculator.
Under CG preset, choose Tailed for most aircraft, or Flying wing.
Under Wing shape, choose Tapered for a wing with one straight taper.
Measure one half of the wing. Enter the root chord, tip chord and half-span.
Enter Tip LE sweep: how far the tip's leading edge sits behind the root's.
Read the CG from the root leading edge, and mark it under both sides of the wing.
Tap Save to keep the result with the aircraft.

28% of the MAC is a safe first-flight target for a tailed aircraft.
For a wing with more than one taper, choose Multi-panel and add a row for each panel.
Tap Measure with camera to fill in the sizes from a photo instead.
Compare the result with the kit or plans. If they differ, start with theirs and fly carefully.

CG: Center of gravity: the point where the model balances. Too far back makes it unstable. Too far forward makes the nose hard to lift.
MAC: Mean aerodynamic chord: the average front-to-back width of a tapered wing. CG positions are given as a percentage of it.
Chord: The width of a wing or control surface, from its leading edge to its trailing edge.
Root: The inner end of a wing or stab, where it meets the fuselage.
Tip: The outer end of a wing or stab.
Span: The width of the wing from tip to tip. Half-span is one side, from the centerline to the tip.
Sweep: How far the tip's leading edge sits behind the root's. Forward sweep is a negative number.
Leading edge: The front edge of a wing, stab or control surface.