A Turkish word can have either all soft or all hard vowels (except for words adopted from foreign languages, like tarih - it has both hard vowel "a" and soft vowel "i", so you know it´s not an originally Turkish word, because it doesn´t follow Major Vowel Harmony). For example melek  is a Turkish word - it has only soft vowel "e" - so you know that the suffix you use needs to have soft vowels - either -e or -i.

What you do with the vowels in the suffix depends largely on the suffix that you use.

Some suffixes are the -a/-e type.

For example when you make a noun plural, you can only add - ler or -lar, there´s no option with -i/-ý/ü/u.

melek - melekler (melek-ler)

Other suffixes are the the i/ý/ü/u type.

For example when you add -li to make an adjective, you can add -li, -lü, -lý, or -lu - depending on the vowel harmony.

süt - sütlü (süt-lü

So look at the suffix, it will tell you what vowel harmony to use.

Here´s an explanation on the use of buffer letters.

As far as when to drop a vowel or use a buffer letter, you need to learn it while you learning suffixes.

For example, the suffix for "my" and "I am" is -im/ým/um/üm.

If you have a word that ends in a vowel, you drop the vowel, for "my":

araba - arabam (araba-m) - my car

and you add the buffer letter for I am:

anne - anneyim (anne-y-im) - I am a mother