The tape is just to hold a number of bunches in place in a line. Imagine gathering enough of the fibres to make something resembling the size of the bristle head on, say, a #3 (modelling) paint brush, then putting a dab of glue on the end to keep them together. This we will call a tuft. Now, let's say that each line we want to make is, for example, two inches long, so we streatch a length of masking tape out long enough to accept this length of line, sticky side up. (It can be folded under at each end to keep it in place on the bench.)
Having 'assembled' a number of 'tufts', these are then placed on the tape, with just the bottom edge (where it's already glued) pressed onto the adhesive of the tape, so probably just the top 1/3 or 1/4 of, say, 10mm Tamiya tape is holding the tuft in place. More tufts are added and, the closer together they are, the 'denser' the line of 'grass' will be.
When the desired 'density' is reached, a line of (un-diluted) PVA glue is run along the bottom of the line of tufts, where they are attached to the tape, and this is allowed to set. Meanwhile, further taped lines of tufts can be assembled, to any desired length.
Once the PVA has cured, if desired, the remaining tape can be folded over, onto the 'open' edge of the line of tufts, or trimmed off. Although relatively delicate to handle, you should now have a 'comb' or 'brush' - looking tuft assembly, which can be picked up as a whole.
[This is where a bit of forward planning comes in, regarding the materials used to make, or cover the base. The base material itself should be rigid and stable, and MDF is ideal for this, being both of the above, as well as easy to work with, cut, drill etc, and light to handle when moving a completed diorama. The covering for this base will be determined by what type of terrain is to be represented, where 'flat' surfaces such as concrete, asphalt, or 'normal' grass can be applied directly. But, where depressions, hollows, trenches etc are required, and, in this case, where objects such as trees, long grass and, for example, sign posts need to be physically planted, then a secondary, top, layer needs to be considered. In many cases, the MDF itself can be drilled, bored, gouged or whatever, to fit requirements, but it's often easier to add a softer surface, such as papier mache, plaster, or, an ideal material, expanded polystyrene, such as an insulating tile, or piece of packaging material (you might know this better as 'Styrofoam' in the U.S.). This can be glued to the MDF base, again using the PVA, and then carved, cut, gouged, hacked, whatever to any desired shape, to create undulating terrain, maybe trences etc etc. It also helps tremendously when it comes to 'planting' objects, as they can be pushed into the surface, and fixed, again with PVA, or CA if really needed for heavier items. This means that the 'ground cover' materials can be added and arranged before any taller items such as long grass, trees etc are planted.]
But, in this case, we don't have that luxury, so attaching the tuft lines can be done another way.
The line, or strip of PVA holding all the tufts together can be used as a base for 'mounting' the assembly onto the base, and also acts as a disguise. If possible, scrape even just a shallow depression into the surface, even if it's just into the ground cover material. Given that this leaves a clear area, the base of the 'tuft line' can be glued along this clear line, which can be straight, or whatever shape is desired, and left to set. The exposed base of the tufts then can be covered with a mix of PVA and the sprinkle material used to cover the base, whether modelling scatter mateial or real earth, and further disguise added by using bits of 'weed' or 'grass' scenic material.
Once all the tuft 'combs' are planted and have set, the individual tufts can then be brushed-out and arranged, which will make them intrtwine, lean one way or another etc, depending on your choice. Thsi will not only hide the base of each line, but, Hey Presto!, you've got what appears to be patches of wild, long grass. This can then all be further enhanced as desired.
I hope this rather long explanation makes sense, but if you need some sketches to illustrate it, just shout !