If you don't already have 2.5" strips you can use this ruler to cut them.
Then place the 2 fabrics (that you want to sew together) right sides together and place the triangle portion on top of them.
You will be able put your cutter into the little circular openings at the bottom of each triange and fully cut both sides. The pieces between the triangles on the rulers are the same size so the triangles are being cut from both edges of the strip at the same time. They are ready to take right to the machine and sew together. Here is a link to the product's company webpage where you can click on the state you live in and see who sells them. A lot of stores will probably just special order these since they are more of a specialty ruler. Here is another link where you can watch videos demonstrating some of them. This company makes a lot of the standard size rulers that we all use too.