Dust off windows with an old soft bristled brush and tack cloth
Pour a small amount of mixed primer, I like Kilz; into a easy to hold container. I chose a rinsed out Greek yogurt tub. Careful to keep your blood sugar in check lest primer begins to look delicious.
Dip nylon/polyester brush an inch into the paint, removing excess paint on side of delicious yogurt container.
Start with the inside frame of the window, starting at the top of the frame. Quickly realize you are not infallible and run downstairs for the painters tape. Mask non-wood window tracks and weather stripping. ... Take a deep breath ... Return to step 3.
Let dry, aka move onto next window, then the next window, then the next window, and when 2 hours have gone by and the constant reminder of yogurt makes your stomach rumble, curse the day you blithely said, "Why would I ever pay the window company to pre-prime my windows? You've got to be crazy to think I'd pay someone to do that!" Quickly realize you are indeed crazy, in fact, you've been living in a diy haze of delusions of grandeur since you began this whole renovation project. What made you think you could renovate two apartments and maintain any sort of sanity or regular sleep schedule. FOOL! All the while attempting to schlep a 7' tall ladder around the room with a paintbrush held between your teeth. Uh oh. Something feels off, a bit wet, slimy some would say. Remove paint brush from mouth. Step away from the ladder. Go down stairs and eat a snack.
Paint should be dry. Raise the lower sash and drop the upper sash. Paint all unobstructed surfaces. Move sashes until all surfaces are painted.
If you are a Bad A*$ and you know it, continue painting up to glazing without the aid of painters tape.
Make a mental note to buy razor blades to clean up the mess your Bad A*$ self just made of the window panes.
Stop before completing all the windows, swear to yourself that you hear the dog barking to go outside. Do not, I repeat, do not go back upstairs to finish painting even once you get downstairs to find the dog in deep REM sleep blissfully unaware of your struggle.
Clean brush out using nifty brush comb and call it a day. Commence cocktail preparation or ice cream consumption, I leave the choice to you. I personally recommend both as you come to the realization that you have only PRIMED the first 3 of 16 windows. Oh holy regret ...
But seriously, if you purchase a quality brush, do yourself a favor and also pick up a painters comb to keep that brush beautiful. It is totally worth the hassle of trying to find one. Big box store, neighborhood paint shop, contractor's supply store - all completely out with no intent to restock. I may possibly be the only person not to have already acquired one. Don't let yourself be thee last.