The trick is to start with the nous form, then drop the -ons ending and add -ant.
Examples:
Manger - nous mangeons - mangeant
Venir - nous venons - venant
Boire - nous buvons - buvant
There are only three exceptions to the rule:
être - étant
avoir - ayant
savoir - sachant
I find the exceptions pretty easy to remember since être doesn't end in -ons in the nous form, and "avant" and "savant" (what you would get for avoir and savoir if you followed the rule) are already words.
as an adjective