How to inculcate metta in me? Metta was natural to me when I was young. I didn't give much thought to it then. 

Now I find it difficult. So many thoughts come when I want to donate to charity like,
is it a recognized charitable institution? how is it run? if they are adequately funded, should I donate to an institution which does not do marketing? or whether I should give the part to the homeless person?   All these thoughts makes metta a mental process of deciding who is worthy and who is not with limited funds I have for donation. 

How does one inculcate metta? Is it influenced by skandhas ? Can metta itself become an attachment leading to mental craving for that feeling when one can observe on giving?
If funds are limited and the opportunities to donate are plenty, you should find out which is the best place to donate as in who is genuine, who will benefit the most, who is the most worthy etc. It's an aspect of wisdom which guides Metta to have the best possible outcome. 

It's important to note the difference between worrying and looking for the best available option. The former is a consequence of clinging whereas the latter is a form of wisdom.

The Skandhas(5 aggregates) is a different teaching which has nothing to do with Metta, except for Metta bieing an instance of Sankhara-skandha(mental formations).

Metta can never be craving, but craving can disguise itself as Metta. 
Ex: Wanting to donate because the collection girl is pretty; wanting to join community service to get a chance to spend time with someone you are  attracted to; wanting to donate to impress someone; wanting to donate to get a heavenly birth etc.

Technically speaking, wanting to help the poor &amp; the afflicted is more Karuna(compassion) than Metta. Metta is simply kindness.