Military modelling is actually a broad subject that can refer to tanks, planes, ships, figures and artillery. All of those things are slightly different but the basic techniques used in building military model kits are fairly universal. You can use a number of "tricks" to make your builds easier and your paint jobs more authentic.
Military model kits are more complex with each passing year, and one of the big mistakes people make is trying to assemble everything in one night. Similarly, modelers will often assume that once the hull and the turret of a tank are both assembled, for example, it is time to put them together. You'll get better results with patience and assembling major parts as recommended by the directions in stages.
Unless you have an incredibly light touch with a traditional brush, airbrushing is the key to convincing paint jobs on military equipment. Airbrushing properly generally requires a controllable compressed air source and shooting the paint at somewhere between 10 psi and 20 psi. Less pressure equals finer lines and more control. Paint across the model in a back and forth sweeping motion approximately 6 to 8 inches from the kit for general coverage and closer for finer lines.
You can get straight lines, for example on the wings of aeroplanes or hulls of ships, by using painters tape or masking tape. For covering other surfaces to protect them from paint, for example headlights or cockpit glass, use a liquid masking solution. You should wait for paint to completely dry, which may mean 24 hours, before removing the mask.
Dry brushing is a technique that adds dimension to your scale models by calling attention to raised details. To dry brush, take a flat headed brush and dip it in paint about 1/4 of the way down the bristles so just the end is covered. Wipe off as much paint as you can and then lightly pull the brush over the surface. Paint will stick there and raise the details. It is best to dry brush with a shade or two lighter than the underlying colour.
Weathering can mean many things, but it is the process of making a military model look combat tested. You can dry brush silver paint to look like paint wear, create mud clumps with plaster filler, dry brush black around exhaust pipes and pierce armour plating with the tip of a heated awl to produce convincing effects. You can powder pastel chalk in rust, brown and black and brush it on for dust and grime as well.
One thing that will really help the final appearance of a model is to create an oil wash and apply it. Seal the entire kit with a flat, clear sealant. A couple of coats is a good idea. Mix a small amount of black, dark grey or dark brown oil paint in a solution of paint thinner in a ratio of approximately 5 per cent paint and 95 per cent thinner. When the paint has completely dissolved in the solution, brush the solution over the entire kit. It has a unifying effect and will improve the overall look and may be your final step.
Decals can sometimes be a real pain if you are not sure how to do it properly. Always cut your decals out with a hobby knife as close to the edges as possible and use tweezers to manipulate them. Prepare the surface of the kit for the decal with a high gloss floor wax brushed on where the decal will go. Keep the water you use warm but not hot. Don't try to do more than one decal at a time, and remember to slide the decal in place gently.