BMarie, Kolya has explaned very well.
I'll try too.
The short version of the prayer is:
G o s p o d y
Lord
(the last letter sounds like 'e' in the word Easter)
Stress on first 'o'
I i s u s e
Jesus - in Ancient Slavonic
(each of letters
i sounds like 'e' in the word Easter,
u -like <oo> in the word 'moon', last
e - is similar to 'e' in the word 'led' and combining with previous 's' makes it softer)
Stress on 'u'
H r i s t e
Christ - in Ancient Slavonic
(
H -l ike in word 'house',
i like 'e' in Easter,
<te> -'e' sounds more similar to 'e' in the word 'led', although not exactly, and makes previous 't' more soft)
Stress on 'e'
P o m i l u y
forgive/ show mercy
(
i -like in Easter,
u -like in 'moon',
y is like a first sound in the word 'your' or last sound in French word 'fille')
Stress on 'i'
Mya
Me - Ancient Slavonic version of the Russian word Menya
(difficult to explane how it sounds,
ya would sound like German Yes- Ja, but connecting with 'm' the <j> almost vanishes and gives its softness to 'm' and 'a')
I've read in one book that Saint Simeon the Divine recommended to use the short version at first.
He says : "Sit down quietly and solitarily, bow down head, close eyes, try to breathe as quietly as possible, look with the help of your imagination inside of your heart, take your mind (thought) from head down to heart. While breathing say: "Gospody Iisuse Hriste, pomiluy mya" -quietly with lips or only in mind. Try to drive off other thoughts, have calm patience. And repeat this often."
(I've tried to translate correctly, but possibly there are some inaccuracies)