Hello members : How do you determine or when is a good time to re-pot cutting from plastic cup to 1 gal pot ? 
Once decided to proceed, what must I do or any special attention such as watering before or after, fertilizer etc. to be carried out. Thanks

Personally I wait to see roots at least 1.5" long, preferably 3.  Then I pre-wet my potting mix so it's damp with lots of air spaces and fluffy.  I put as much wood under ground as possible without letting any roots touch plastic.  I put some soil in, put the plant in so the roots barely touch and gently fill in the soil.  I don't tamp down.  I water when the top inch or 2 is dry and use a spray bottle to mist the surface for the first few waterings.  Depending on your mix that may be a lot of pumps  :)  If the plant looks happy I start fertilizing with half houseplant strength and slowly work my way up to full houseplant strength.

Thanks for the quick respond. Can I use 50% Peak Moss and 50% organic potting mix ?
Do I have to pre-water the 1 gal pot with potting mix completely one day earlier or what ?

It depends on what the organic potting mix is like.  You want something that drains quickly and has lots of air space.  You don't want something that turns to mud when you water it.  I suspect the organic potting mix may turn to mud.  You can search for posts by Tapla or search on 5-1-1 mix.  I use promix HP with biofungicide and mycorrhizae mixed with pine bark chunks and compost.  There's a little espoma and worm castings thrown in also.

Whatever mix you choose you make up a batch of it and get it wet enough so it just barely holds together when you squeeze it.  If you can squeeze water out of it it's too wet.  As you fill your pot you fluff up the soil and don't compact it.   put an inch or so of soil in the new pot and place your plant in, then fill around the roots, keeping the soil fluffy and loose, not tamping down.  When it comes time to water, do so very gently.