Hi! If you are writing in Quenya [with the Elvish letters] and you have a word that begins with a short vowel, then I would place the vowel mark over a short carrier -- a short carrier looks a bit like an 'undotted' i.

If an English word, then I would place an initial vowel over the following consonant, although you could do that, or use an initial carrier as well -- if you generally went with 'preceding consonant' I mean. I would be consistent in the same text however, depending upon your choice.

Tolkien will later describe a 'general use' mode applicable to Sindarin and Quenya, where vowel marks may in this mode be placed either above the following or the preceding consonant tengwa. And one can write English in the 'general use' as well.

Appendix F notes: 'When there was no consonant present in the required position, the tehta was placed above the 'short carrier', of which a common form was like an undotted i'

Long vowels could be placed upon a long carrier [like an undotted j], or they could be doubled, which was however only frequently done with the 'curls' and sometimes the 'accent'.

For initial long and short carriers, see Tolkien's usage in the following Quenya transcription...

http://folk.uib.no/hnohf/namteng.pdf