There's a couple ways of going about this. I'm going to show you my preferred method: editing a clef to create a fake key signature next to it. The great advantage of this is to keep spacing around the beginning of a staff consistent and therefore cleaner. However, it is less flexible when it comes to more complicated situations where key signatures changes a lot. In those cases, using symbols and manually putting the accidentals in is usually better. But this is very dependent on the music's complexity.
To do the "edit clef" method:
1) Have your notes all ready to go with accidentals in place. See our default example below.
2) Go to House Style > Edit Symbols
You will get the following dialog box:
3) Locate the "Clefs" section.
The boxed area in red are the clefs that will show up when you press "Q" (or Create > Clef) to create a clef. You will have to edit one of these for it to be usable. I edited the one circled in green, the old-style double treble clef. Those are so rare nowadays that its usually safe to edit. Remember, this is a House Style edit so it will follow only this score; it is not a global change to the defaults.
4) Click "Edit" and you will get the following dialog box:
Remove the old clef by selecting the symbols from the box below and deleting them.
Be sure to give it a name.
5) Locate the main clef you want to use, in this case, treble clef. This is your "main" symbol.
6) Adjust the spacing so that it is centered on the G line.
Note that a "space" is the amount of distance between two lines on a staff. A value of "-1" will center it like a normal Treble clef.
7) Click "Add" to add the flat.
8) Locate the flat in the list of symbols. Hit OK.
You will see that it is added to the list of "Other Symbols". Notice too that it is overlapping the treble clef. We need to space it out a bit.
9) Adjust the spacing as desired. For a lone A-flat, values of "-0.5 up" and "3.5 right" work well. Hit OK.
10) Now you have a new clef. You'll see it updated in the clefs dialog by pressing "Q":
Select it and hit OK. Then place it in the score.
11) Here is what we have now:
Remember the key signature is fake, so flats will not automatically go away yet. We need to do this manually. We also need to fix the spacing.
12) Fix the spacing by going to "House Style > Engraving Rules"
13) Select the "Clefs and Key Signatures" page and adjust the "Initial clef width" to a larger number. The default is usually 3.5. I used 5 spaces in this example. Hit OK.
You will now get this:
14) Hide those accidentals by selecting them and hitting CTRL + SHIFT + H (or "Edit > Hide or Show > Hide") You will get this:
Hidden things will not print by default. Turn off hidden objects by hitting CTRL + ALT + H (or "View > Hidden Objects"). Check your spacing and make any adjustments as necessary. The final score in the example would look like this:
Hope this helps! Good luck!