Don't be afraid of asking questions here on TBNet...STUPID is when you break something expensive rather than asking for help...
Check the operators manual for the machine...if you don't have one, check online...and check on the cowlings of the machine itself...my old 870 John Deere has a plate showing RPM and gear / ground speed combinations...
OR...in absence of a ANY manuals or plates etc...the light is not as important as the "feel"....start in a low gear range, the tool raised, at low RPM, then engage the PTO and let the tiller start to spin unloaded, bring up the engine speed a little and slowly lower the tiller into ground contact, then see what happens as you move forward....experiment a little....experienced as you are with manual transmissions, you will soon feel the "right" combination of gear, ground speed and engagement depth...you should neither lug nor rev...and feel good about helping out your neighbor...she must be a sweetheart to make you want to spend your time in the dust and heat...
Above all....
Be CAREFUL out there!
Terry