I know I can use simple present for instructions. But, If I want to explain how to do something now, can I use the simple present?
For example is it correct if I said
"I show you how to do it."
"I'll show you how to do it."
Using the present simple:
I show you how to do it can be used
as a caption for a photo, or a chapter title in a book (or a title of a book).
You can also use it when you are going over the stage directions of a play or movie and walking through the scene and using the simple present to describe each action:
I enter the room, I grab the gun from the guy's hand, I show you how to tie the guy up, I call the police...
but again, this is an extremely restricted context.
A similar usage of the simple present is when an announcer of a soccer game calls the action at the moment it happens: Michele kicks the ball to Louis, Louis shoots and scores. But, frankly, it is hard to think of a context when this exact verb show how can be used in this sense. Unless there is some TV show that includes a step or action show how to, as in Janny shows John how to throw a frisbee.
The demontration present is used on cooking shows (see TRomero's example).
There is also the present tense that you use that actually causes what you say to happen, such as I name this baby Dino or I forgive you. But show how doesn't easily fit these use.
A usage it does fit is whan talking about the future.
First, every statement about the future is a prediction. This is because we cannot be 100% sure about anything in the future (or in future time, or when speaking of future time).
The sentence with the modal will (I will show you how to do it) is one that is more common, and you can use it in many more contexts. The sentence in the simple present (I show you how to do it) is valid only in a certain contexts. I now explain that context.
The strongest prediction you can make about the future is by using the simple present:
1 The train arrives at 10am.
2 Tomorrow I go to Switzerland.
3 The sun sets at 8pm.
4 The world ends in five minutes.
Even though your prediction may be wrong (as #4 will hopefully be), you can still make a prediction (that is: talk about the future/refer to future time) using the simple present. This is how to make the strongest prediction about future time. It is similar to a time table of events, or a schedule of events.
So, you can use Sentence 1 in this same way:
5 I show you how to do it in five 5 minutes/at 8pm/tomorrow.
This is a very strong prediction about the future. You are talking about the future as if it were a schedule of events.
As such, you can also use this sentence when explaining a series of steps that you plan to do. For example, a teacher can say:
6 Today in class we will do a lot of things. And here is my plan. First, I show you how to use the subjunctive. Then, I show you how to use subordinate clauses, then I show you how to use certain idioms, then I show you how to choose good songs to listen to.
Two things to notice:
A. The teacher states his/her plan in the simple present. In doing so, they are talking about future time with the strongest possible sense that they think what they say about the future will come true. In other words, this is the the way to predict things about the future with the strongest preduction. It talks about future things as a schedule of events that will come to pass. (Even though, technically, there is never a 100% chance that they will.)
B. The teacher could also use "I will show you how" in each of those sentences, but the prediction about the future is not as strong. In addition the meaning of will is a bit different.
7 I will show you how to do something,
the use of will conveys the speaker's present intention to carry out an action in the future. It is a promise.
It is very easy to use Sentence 1 incorrectly. This is why it sounds ungrammatical when it is written alone, with no context.
All the examples found on Page 1 that Lucian found use Sentence 1 in the same way I have described.
Note also, my sentence
8 "I now explain that context"
is another example of this usage.
Your first sentence sounds off, the second one is definitely correct.
To explain something you're about to do, you normally use going to or will. If you planned the 'explaining' before the moment of speaking, you use going to:
I'm going to show you how to do it.
However, if you did not plan it beforehand and you only just thought of doing it, you use will. If you see someone doing something wrong for example, you can use:
I'll show it to you.
In case you're already in the process of showing it, you say:
I'm showing it to you.
Since the verb to show implies an action that takes some time to complete, it is not quite commonly used in the simple present. However, you could use it in conditionals:
When I show you how to do it, you will see that you're doing it wrong.
It also works in a different meaning when expressing a fact:
His body shows signs of cancer.
In the above sentence a present continuous might be preferable though:
His body is showing signs of cancer.
I cannot think of a way to make the simple present work in your sentence while retaining the same meaning of show, I suppose because the verb expresses an action and actions are normally not used in the simple present unless it is a repeated action.
You can find more information about the use of the simple present here.
I show you how to do it.
is grammatically correct as pointed out in DamkerngT.'s comments.
Also please see this:
Can I use simple present to explain how to do something now?
Yes, you can explain how to do something using the present, like this:
One takes (or 'you take') the green triangle and places it in the orange container. Then the blue square goes into the red bucket. Then one rings the bell. The first one to ring the bell wins.
This is how the simple present is used when one explains how to do something.
The present tense of "show" is natural in conditionals: "If I show you how to do it now, then you'll be able to do it yourself tomorrow."