I'll keep it simple. Bewared that my method may not be the best. Also it's not a good idea to do this if you are planning to sell your Valks.
These are what I used for panel lining and coating. Make sure you use them in a ventilated area and wear a mask!
A general rule is to use grey on white surface, and black on others. I use both Tamiya and Gunze panel liner. Tamiya is fine but DO NOT use it for weathering wash. It will crack plastic and destroy polyethylene (ex. Gunpla joints). Use mineral spirits (or enamel thinners like Tamiya X20) to wipe off the excessive afterward.
Gundam markers are very useful, especially on Arcadia products. Arcadia has shallow panel lines which make the liquid panel liners useless.
After applying the Gundam marker:
1. Use eraser to erase the unwanted marker (magic!)
2. Use Mineral Spirits (or any enamel thinner) to soften the color and blend the edge
3. Use lacquer thinner and fine tip cotton swaps to clean up the details
Occasionally, I'll scribe panel lines to better resemble the line arts. BMC 0.15mm and 0.3mm are what I use the most. Make sure you're using the scribing tapes.
After all the hard work (use water slide decals if possible) it's time to coat. I use Tamiya flat (TS-80) on most models and I like it better than the Gunze's.
Use masking tape to cover places like canopy and sensors. Find spray paint instruction videos on Youtube. I'll only tell you some tips here:
1. You can use a hairdryer to greatly speed up drying
2. You can warm the spray can with warm water to increase its internal pressure
3. Wear a mask and spray in front of a ventilation booth. You're more likely to catch dust if spray outside, say, in the garage
4. Apply a thin layer of coat first. You may notice a bleeding effect if the first application is too thick (apparent on Arcadia products)
5. Be patient. You'll need to partially transform and loosen your Valkyrie to coat all the surfaces. It can be tedious and things tend to break at this point.
Here is a before and after comparison. The details are accentuated and look less "plastic-ky". BTW stickers are acceptable if their boarders are trimmed, like the "02" here.
That's it for now. Happy modeling and hope this helps!