A wide variety of tools work quite well. A much cleaner edge than you can get from your fingers. I still like to blue tape the edges before using these tools as the silicone just smears over the tape which you can peel off and then just tape over the small edge the tape left.
High Tensile Strength Duct Tape Silver, 5.8mils, 2 Inches x 50 Yards - All-Weather Duct Repair Tape for Commercial Use, Heavy-Duty Repair, Packing with Temperature and Shear Stress Resistance - XFasten
39 Review
Transparent Cross Strapping Tape: XFasten Filament Duct Tape 2X30 Yards - 3 Pack
29 Review
TapePlus Gaffer Tape - Large 2 Inch X 40 Yards Roll - Versatile Black Tape For Gaffers, Ducts, Floors, And More
31 Review
Gorilla Crystal Clear Repair Tape, 1.88β X 9 Yd, Clear, (Pack Of 1)
32 Review