am planning on cosplay nami or female laharl and they both wear bikini top and want to now what the best way to had what i said in title and would like to now what the best think to do thank u

There are specific brands of concealer than can be used for that sort of thing (see http://voices.yahoo.com/best-products-cover-birthmarks-3852312.html ). If its an area that will rub off against clothes I'd also fix it with a fixing spray or powder.