German selber and selbst

When selbst precedes the noun or phrase to which it relates, then it has the emphatic meaning “even”, as an adverb. In other positions, selbst is completely equivalent to selber, that is, a demonstrative pronoun translated as a “-self” word. In this meaning, you again need to pay close attention to position in the German sentence. When it immediately follows an object, then it’s emphasizing that object, just like in English when you place a “-self” word immediately after an object.

1. Demonstrative / Intensive Pronouns 

Both “selber” and “selbst” can be used as demonstrative / intensive pronouns. When used as intensive pronouns, they are perfectly interchangeable. 

An intensive pronoun is used to put emphasis on another pronoun in a sentence. In English, intensive pronouns end in “-self”.
Example:
She cannot do it herself. 
Sie kann es nicht selber/selbst tun/machen. 

In English and in German, an intensive pronoun can be moved in a sentence to modify the emphasis. We can place the intensive pronoun immediately after the subject or subject pronoun. 

Example:
Sie selber/selbst hat das Buch geschrieben. 
She herself has written the book. 

In English, we can also place the intensive pronoun at the end of the sentence, which doesn’t always work in German. If a German sentence ends with an infinitive verb, a separable prefix or participle, “selber/selbst” cannot be at the end of the sentence. Often “selber/sebst” is placed immediately following the conjugated verb, and/or after direct or indirect objects. 

Example: 
Sie schreibt das Buch selber/selbst. / Sie schreibt selber/selbst das Buch. 
She writes the book herself. 

BUT:
Sie will das Buch selber/selbst schreiben. 
Sie wants to write the book herself.

Sie hat das Buch selber/selbst geschrieben. 
She has written the book herself. 

2. “selbst” as an Adverb 

“Selbst” can also be used an an adverb, meaning “even” or “not even”. “Selber” cannot be used the same way. 

Examples:
Selbst mein Kollege hat es vergessen. 
Even my colleague has forgotten it. 

Selbst wenn ich keine Zeit habe, versuche ich zu kommen. 
Even is I don’t have time, I will try to come. 

In neither of those two examples can we use “selber”. Only “selbst” can be use an an adverb. 

3. “selbst/selber” + past participle“ as an adjective

Another way “selbst” may appear in a sentence is as an participle adjective. The best way to translate a “selbst + participle” adjective is “home + participle”, as in “homemade, home baked, home cooked, etc”. 
Before the German spelling reform, those were usually spelled as one word. Today, they can be spelled as one word or two. 

Example: 
Das ist eine selbstgemachte /selbst gemachte Marmelade. 
This is (a) homemade jam. 

Der Kuchen ist selbstgebacken / selbst gebacken. 
The cake is homemade/baked. 

Be careful, however, with participles, as they can also be used in the formula that constructs the present and past perfect, and the passive voice. 
In those case, we are not using “selbst + participle” as an adjective, but as an intensive/demonstrative pronoun, which means we can also use “selber” 

Example:
Ich habe die Marmelade selbst / selber gemacht. 
I have made the jam myself. 

Ich habe den Kuchen selbst / selber gebacken. 
I have baked the cake myself. 

4. “Selber!“

Another example where “selber” and “selbst” are not interchangeable is when “selber” is used, mostly by children, as a means to return an insult. 

Example: 
Du bist langweilig!   – Selber!
You are boring!   -You are!

5. Von selbst/selber

When used with the preposition “von”, “von selber/selbst” is used to express “by itself”. We can also say “von alleine” (lit. by alone). 

Examples:
Das Problem hat sich von selbst/selber/alleine gelöst. 
The problem has been solved by itself. 

6. Common Idioms with “selber/selbst” 
There are a handful of German idioms that use “selber/selbst”. 

Examples:
Selbst ist der Mann/die Frau. 
lit. himself/herself is the man/the woman. 
Meaning: Do it yourself./ Help yourself./Take initiative. 

Alles muss man selber machen!
lit. One must do everything oneself.
Meaning: No one is helping, so one is forced to do something independently. 

Das glaubst du doch selber nicht!
lit. Not even you yourself believes that!
Meaning: Used when someone says something so outrageous, it is hard to believe they believe what they are saying. 

Be careful not to confuse these two words with the similar-looking demonstrative pronouns dieselbe etc. When selbst precedes the noun or phrase to which it relates, then it has the emphatic meaning “even”. In other positions, selbst is completely equivalent to selber, that is, a demonstrative pronoun translated as a “-self” word.