The L4D style signs that point to a certain point in the map are done with the point entity "training_annotation".
You can copy them from the decompiled map if you want a ready made one, but from what I can see, there's only 3 important keyvalues and 2 inputs.
training_annotation
targetname - to 'name' the entity in the input/output system.
lifetime - the lifetime of the annotation (set it to -1 for it to stay visible until commanded)
display_text - The devs used calls from the tf_english file to get the instructions in the training maps, but your basic text will work here too (I think if you type "\n" you get a new line, but I've not tested it).
This was in the tf_english file:
//training
// Some of these training strings have control characters embedded to set colors.
// The control characters can be copied and pasted in notepad.
// = 0x01 (SOH) - Use default color from this point forward
// = 0x02 (STX) - Use hint color from this point forward (HudTrainingHint in ClientScheme.res)
You can also call pre-written lines in different languages by putting stuff into the
display_text field, for instance if you put:
#TF_Training_SpaceToContinue
it would show in game as
"Press Spacebar to continue"
you can find other calls at the bottom of the tf_english text file in the 'team fortress 2 content.gcf' under the resource folder if you want them to support multiple languages at the cost of less flexibility.
The two fairly simple inputs are simply "
Show" and "
Hide" though they'll come up as broken in the hammer editor (due to the lack of a new .fgd) they'll work in game fine.