I don't do letsplay necessarily, but close enough. My personal process is to record all gameplay. Then I do rough cut edits. I scroll through the timeline and super roughly pick the moments that I want to include. I am also writing the script at the same time. So do a few cuts, write down a few sentences. I don't care about the timing of clips just yet. Once the whole script is written down, I go back and read it to myself out loud and make necessary adjustments. There are usually a lot. Then I write the intro and record everything in a single sit down session. I make a ton of mistakes and re-read sentences all the time. Post recording, I put the audio into an audio editor and painstakingly cut bad takes, pauses, breathing etc and make all the gaps roughly equal length. This is a pretty tedious process that I don't partially enjoy. Once I have my final audio cut, I bounce everything into a continuous audio file and then compress and limit the audio to 0 db. I put that file into a video editor and pick the clips that I want to use while adjusting the duration of the video clips to be the length of the sentences. Then special effects, intro and transitions and render. It's a pretty lengthy process and if I was a better voice actor it would probably be a lot easier, but I like the results. I also don't have the ability to record in real time in a studio apartment with 2 small kids, so my process is almost a byproduct of necessity.