The 5050 LED size fits perfectly on the PCB breakouts. Since there isn't an alternating warm white, cool white strip available, we have to create this relationship by hand.
The first step is to get a steady surface. Gravity does come into play when using solder paste.
Whenever you're using a heat gun, it's important to use a surface that won't scorch (or ignite!) so take care to select something appropriate. Cloth is not appropriate. Wood is also not appropriate. It's going to get very hot.
In my case, I used an aluminum baking tin from the grocery store. I turned it upside down, and set it on something that doesn't react quickly to heat. I used the flat surface for my PCB.
Before doing any SMT work, I use chemwipes and rubbing alcohol to wipe down the copper pads. This cleaning helps the solder paste adhere smoothly.
If you don't, your mileage may vary.
I did attempt to solder one of these by hand, and failed at it. The pitch is extremely fine. I wouldn't recommend it.
Solder paste often comes in syringes with thin metal tips. The thinner the tip, the better. You want to use as little paste as possible to get the job done, to reduce waste and discourage solder bridges forming.
Keep the 5050 PCBs attached to their grid.
Using a steady hand, apply a thin line to each row of pads on the PCBs. Take your time. You want to make a single pass, and you want to distribute a very thin line of paste. In my project, I used far too much with each pad, but I didn't have a thinner tip available.
If you make a mistake, and accidentally cross lines of paste between the two sides of pads, use alcohol and chemwipes to clean the paste off that pad and start again. It'll save you a headache later.
Making sure not to mix and match the cool white and warm white, use tweezers to gently set the LEDs onto the PCBs. The orientation is extremely important, so take note where the small corner indent is on each LED, and keep it consistent to ensure you line up the pads, and don't accidentally reverse the orientation.
You don't have to press the LEDs into the paste, they'll naturally attract in the next step. If you accidentally get solder on the top of the LED, or flip it over, remove it and clean it with alcohol and chemwipes, and then apply it again.
The first thing when using a heat gun is figuring out your exit strategy. Once the gun is used, it's going to be very, very hot. Have a plan on where to hang it, or set it down, so that it can cool off. It's hot enough to melt solder, it's hot enough to melt a lot of things. I usually use a hook and hang it in my shower on the rod (away from the curtain.)
Holding the heat gun downwards, apply a low amount of air to the PCB. I keep it as low as possible so I don't blow the LEDs off the paste. Keep moving the heat gun back and forth across the entire PCB to evenly heat up the entire piece. If you stay in one spot, you'll scorch and melt the LEDs.
At first, your solder paste will entirely liquify and then turn solid. This may slightly rotate the LEDs, but as long as they don't get too far off the pads, you're fine. Just keep going. You'll think the process isn't working, and that something must be wrong, but keep going.
After somewhere between a minute and three, the paste will turn from a solid grey matte to a liquid silver, and the LEDs will be sucked into position on the pads by the now molten solder. (Magic!)
Not all of the pads liquify at the same time, so once a few do, try avoiding them with the air to prevent them becoming too hot.
If one of the LEDs gets out of position, use tweezers to gently lift and replace the LED onto the pads in alignment. Apply heat shortly to making sure the solder doesn't form a cold joint.
Now, let both the PCBs and the heat gun cool. Do not move the PCBs while they're cooling. They'll remain molten/soft for a minute or more, and then be enough to burn your fingers for 10 minutes or more.