Paint small parts, interiors etc first, ensuring that those areas requiring cement are either left 'bare' or the paint scraped off. Much of this painting can be done with a brush, although the use of the airbrush is personal choice.
Mainly, it's better to complete the main construction, leaving delicate parts such as landing gear, props and small antennae etc until after painting. Canopies can be masked and fitted before or after painting the main model, depending on type of fit and ease of access.
Mask all open areas of the model, such as cockpits, wheel bays, intakes etc, using Tamiya type tape, and damp tissue where needed in large open areas, ensuring that the tape edges are well sealed down - sprayed paint 'mist'
will get where smoke can't !
It's often better to apply a thin primer coat of a light or medium gray first, depending on the surface and colour of the plastic, and when using acrylic paints, this is normally essential anyway.
Let this primer dry and set for at least 12 hours before spraying the main colours.
As a 'rule of thumb', light colours should be applied first, again allowing each coat to fully dry before masking and / or spraying the next colour.
Once the model has been painted, and then had a gloss clear coat which is allowed to f
ully harden, the decals can be applied.
When the decals are fully bedded down and set, a further 'sealing coat' of clear gloss is applied, followed, once dry, by the desired finish clear coat.
With all of the paint, clear coats and decals fully set, the remaining 'sticky out' parts can be fitted.
The pic below shows my current build ready to be masked for spraying.