I started to watch "How iI meet your mother" series to learning my english.

I plan to watch an episode every single day.

I will look up unknow words in a dictonary during each episode.

iI can parctice it on this application.

I started to watch a series

After “start”, the -ing form sounds more natural in general English but “Started to watch” isn’t wrong, but it’s less common in everyday speech.

I started to watch the series "How i meet your mother" series to learning my e English.

– “Started watching” sounds more natural than “started to watch” here.
– After “to”, use the base form of the verb (“to learn / to improve”).
– Remember to capitalize “I” and “English”.

Good choice of series — it’s great for learning natural English!

I will look up unknown words in a dictionary during each episode.

unknown is the adjective form

I can save unknown words after i did it

i can paractice it on this application.

I started to watch "How i meet your mother" series to learning English. (or to improve my eEnglish.)

I plan to watch an episode every single day.

I will look up unknown words in a dictionary during an episode.

I can save unknown words after i did itI looked up for them.

"did" is ambiguous, it is unclear what you actually did here.

iI can paractice itusing them on this application.

I started to watch "How i meI Met yYour mMother" series to learning my e English.

Titles should have most words be capitalized.

I will look up unknown words in a dictionary during the episode.

I can save unknown words after iI did ito this.

iThen I can paractice it on this application.

App sounds more natural.

I've started to watching a series

I've started to watching the series "How i meI Met yYour mMother" series to learningto practice my eEnglish.

I plan to watch an episode every single day.

I will look up unknown words in a dictionary during the episode.

I found athe perfect application for it.

You could also use "app" instead of "application".

I can save unknown words, and after i did itdoing so I can practice in the application itself.

Connecting this sentence with the next one.

iI can save unknown words, and after doing so I can paractice it on thise application itself.

Connecting this sentence with the previous one.