What is elision? It means 'sound deletion', and if English or French is your native language you will use it every day. Everytime you say "I'm" instead of "I am" or "Je sais pas" instead of "Je ne sais pas" you are using elision - deleting sounds and contracting words. This makes your speech more informal, but also often makes it easier to pronounce.
In Sindarin elision can be found in some of the most common everyday phrases, that have been used so often by the Elves that they have begun to slur them. Inn dha v'im = I have a good mind to (literally 'There is a mind in me'). The contraction here is the words Vi = in and Im = I, myself which have come together to make v'im = in myself. Mae g'ovannen! = Well met! (literally 'You are well met'). The contraction here is from Ci = You are and Covannen = Met (passive participle of Cova- = to meet).
You also use this everytime you use In = The/Of the (plural) or En = Of the, when you write I selaib = The herbs instead of In selaib. In these instances however elision is not optional; In selaib is simply incorrect. An Elf would know what you meant, but it would mark you as a non-native speaker of the language.
Now that you've learnt what elision is, how can you use it in your own Sindarin? As we can see from our attested examples, these involve a pronouns and a preposition, or a pronoun and a verb. Tolkien changed his mind many times on the derivation of Mae g'ovannen however, so I would advise you to treat this as a single occurrence.
Be sparing with the usage of this, and think carefully when you use it whether someone would understand what you are saying.
Examples: N'im = To myself instead of Na im B'ech = According to you instead of Be ech