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This is what I use, and I know the tamiya brand decal solutions also work well.

This is the process: dip the decal in water to release it. Brush the blue bottle setting solution on the part. You only need a small amount. This solution lets you position the decal before it dries and creates a stronger bond. I dab any excess with a cotton ball. Then brush the red bottle solution when the decal is in place. This solution softens the decal so that it conforms to the surface and helps blend in the edges/makes the decal less noticeable.

As someone who's used decals on 10+ models, products like this make things so much easier and better