If you're going to do it in Illustrator, then it's best to have two documents with the same dimensions and guides.
Create a new blank document of 8.5" x 11" landscape (bearing in mind you probably won't be able to print to the edge of the paper).
Turn the ruler on (apple-R) and pull vertical guides out to 3.541" & 7.269" respectively. These are your two folds. The panels are not of equal size because they can then fold into each other
Run small grey (20% black) lines top and bottom of the page on each guide, very fine (0.25pt) and short (about 0.2"). These are your fold guides. You can remove them later if necessary.
Then 'save as' and call it <filename> Outer.
Remove the guides and fold lines and save as again as <filename> Inner.
Lastly save the document as <filename> Outer Blank this leaves the template for you to use another time.
(You can skip the next step if you have a good visual memory.)
Then take a blank piece of paper and fold it the way you want the final artwork and make a note in pencil on it where the panels end up, numbering them. The front panel on a roll-fold is the right-hand of the 'Outer' page, then comes the left-hand panel of the 'Outer' page, then the inside is the 'Inner' spread. The centre panel of the 'Outer' page is the back of the leaflet.
The 'Outer' file should look like this, with the small fold lines at the top and bottom of the page along each guide:
You'll need to experiment a little with your printer and folding to get the orientation right on the second pass and when you know which way the paper goes in for the second pass, then its easy from there.