Preface: this guide requires your game of choice to have a record and playback function, if that does not exist then it will be much harder for you to use this information effectively
Finding Solutions to Specific Offensive Sequences
If you want to test how an interaction works, you first need to decide what the interaction actually is, then toy with it. For example, let's say you're playing 
Sol in Guilty Gear Strive, and you see 
May spamming this dolphin
Guard:
AllStartup:
7Recovery:
11 (24 OH)Advantage:
-3 looking move over and over in a block stringA series of multiple attacks that work well against a blocking opponent. Generally, a block string will be composed of several plus on block or cancelable attacks performed in quick sequence. When blocked, these will slowly push your character out of range and prevent the opponent from counter-attacking, which makes them a very good default, low-risk option when trying to apply offense. and it seems to be getting you counterhitHitting someone while they are in the startup of an attack. In some games counter hits get enhanced by dealing more damage and giving more frame advantage, which can open up new combo possibilities..
It helps if you bind a Play and Record button in your controls since this lets you quickly do Dummy recordings.
Okay so we know May did thisGuard:
AllStartup:
7Recovery:
11 (24 OH)Advantage:
-3 so lets first find it on the command list located in the pause menu, looking at the video included next to the move can help you figure what move they did as well, if the game you are playing does not have videos on the command list, then try all of the opponents inputs or look at their wiki entry.
Once we have figured out what move the opponent used and how it was used we can record it then keep playing our recording, and just mess with buttons until we find something. Let's try some things, see what works, and decide what we think are good options here.
Below is a visual example of the labbing process:
Congratulations, you have now labbed an interaction and learnt what works in a situation (note that since you don't know everything that's about to happen, this is only one facet of it, the more you test the more whole your understanding becomes)
Testing Reactions On Mixups
Let's say we wanna lab something like reactions, in this case lets say that you are playing 
Gran from Granblue Fantasy Versus and 
Charlotta keeps
knocking you down and running a mix-upTo perform a maneuver that forces a defending opponent to choose between two or more options. If the opponent chooses incorrectly, then they are hit by an attack. There are many different types of mix-ups such as crossup(left/right), high/low, and strike/throw. between a low and a high with H Noble Strategy
Guard:
Startup:
26Recovery:
Advantage:
that seems very hard to block.
Lets assume we know what moves she did. First we set up two recordings, one where Charlotta knocks you down with 2U
Guard:
LowStartup:
9Recovery:
18Advantage:
-9 into 22HGuard:
Startup:
26Recovery:
Advantage:
> M
Guard:
High/AirStartup:
17Recovery:
Advantage:
-3/-3/+1 in the corner, and one where Charlotta knocks you down with 2UGuard:
LowStartup:
9Recovery:
18Advantage:
-9 into 22HGuard:
Startup:
26Recovery:
Advantage:
in the corner. Then set them to random playback, then continue to play the recording to test if you can block both options. (you can also likely adjust how likely each recording slot will happen, so if you want it to happen rarely, this can be a good way set that up) We can then press Play, try to react, and keep going to practise it or test how reliable reactions can be
And with that, you will have labbed out reactions.