To convert to custom text, you select the trigger, and go edit--> convert to custom text.
If you want to do it in the GUI, its going to be really clunky.
Bascially you want to have a periodic time trigger that executes every few seconds. Play around with this condition to see if your guy doesn't have any orders.
-
(Issued order) Equal to (Order(stop))
then bascially, have a bunch of if/than/else statements that tell him what to do, make sure you have skip remaining actions in every single then statement at the end of it.
So
if your guy has under 20% health
then
order him to go back to base
skip remaining actions
else
nothing
If there is a unit within 1000 of your unit
then
order your guy to attack that unit
skip remaining actions
else nothing
just an example right there poorly written, but you get the point.