I just painted my bootstripe in the yard on a sunny day. There was nothing easy about it....
Here's what I did:
I experimented with more and more thinner until it was too runny to stay in place. I found the happy medium was about 15-20% thinner. Notice also as you pour your paint+thinner mixture into your pot the thinner will evaporate from your pot very quickly. Swirl or stir occasionally and if it's not as thin, keep adding more as it evaporated.
I rolled it on in VERY short sections. 10" bootstripe and 2-3 feet at a time only. Immediately it would bubble up like crazy (thinner boiling off). I would wait 20 seconds or so and then brush over it in one direction then again in another. No paint on the brush, just cleaned with thinner and shaken dry.
Repeat the process in short segments. Roll, wait a few seconds, during which time you dip brush in thinner and shake it out. Brush clean brush over paint to remove bubbles, brush over it again.
I needed 2 coats for coverage and it was a pain but it looks really really nice. I knew the job was good when I was looking at my stripe and could recognize the yard boss walking behind me from the reflection in the paint.
If at all possible paint in 50-70deg. Early morning, if there isn't dew is probably best.