There are a couple of ways to do tas. Segmenting and building frame by frame. Segmenting is less work when compared to the latter and will also be less optimized in most cases. You can choose your style or you can do both in a single demo as well. I'll let you decide. Also I'll assume you have basic knowledge of demo recording and you know what lmp files are.
You can use prboom plus for this. By using the -recordfromto command line feature you should be able to re-record from a point of your choice. For example
-recordfromto demo1.lmp demo2.lmp -skipsec 30
will record from where demo1 stops into demo2 after skipping 30 secs of demo1.
This method will also make use of slow motion recording (by slowing down the game. You can do this using +, - keys in prboom plus) to further optimize the demo. TAS also allows you to turn on permanent strafe 50 in prboom plus settings which makes your run even faster. Again its about personal preferences.
Please note using ANY ONE of the above makes your demo TAS!
Building frame by frame:
You can use doom replay editor for this. Currently its xdre2. Rodster has a post detailing how to use this: