|An underlay is a piece of paper placed under the fabric that you are going to cut. A roll of plain brown kraft paper works fine. Try to find paper that is as wide as your fabric or your table. If you can't get the width you need, just add on as required. The paper must be as wide as what you are cutting.
An underlay is absolutely required for the best results in cutting very slippery fabrics like silk charmeuse and silk chiffon. Place your fabric on the paper after making very sure that you are starting with a straight grain. Its OK to cut multiple layers with an underlay, just make sure that all are correctly started on the straight grain.
If it's a fabric you can tear, like chiffon, that's the easiest way to establish the straight grain. Use a t-square to draw a perpendicular line across your paper and align the torn edge along the pencil marking.
If you can't tear it, like charmeuse, you have to pull a thread all the way across (nerve-wracking I know) to get the grain to reveal itself.
Once your fabric is laid out on grain and smoothed absolutely flat ( a c-thru ruler brushed lightly across the surface to eliminate bumps can help) make sure to place weights on the fabric to keep it that way.
Place your (nicely smooth, no wrinkles) pattern on the top and use weights to hold the pattern down. Do not pin! Pinning will distort the fabric. There's tips elsewhere on what you can use for weights, cat food cans are fine, anything will do. I use heavy tape dispensers, staplers, anything that comes to hand.
You don't want to move anything once its laid out, so try to get everything laid out in the block of fabric on the table that will fit. If you can't get all the pieces on it, you will have to lay out another block to cut the rest.
Also please note, it doesn't really work well to cut on the fold when using an underlay, although some have been known to cheat. You risk going off grain though and may have to live with the consequences.....So no half patterns! If your pattern piece is meant to be laid on the fold, then take the time to make the other half of the pattern by tracing it off on a folded piece of paper.
Cut out the paper along with the fabric. (It's OK, I know they told you never use your fabric shears on paper, but when you cut fabric along with the paper it works fine. I have a pair of huge 12" Wiss shears for almost 25 years now, and I rarely sharpen them.) Since the paper is lifting off the table and supporting the fabric, the fabric does not shift.
Make sure your weights are not too close to the edge of the pattern, that makes it more difficult to get a clean edge. For those tight corners and curves you can actually flip over the paper that's already been cut and get in closer. That's after you've been practising awhile!
A bonus of using an underlay is that it can become a record of what you've cut. If you are adjusting a pattern on the table, or maybe putting together bits of various patterns for an entirely new garment, an underlay can become your pattern. Just be sure to mark the grainline, and pencil in what the name of the pattern piece is and how many to cut.
I don't use electric scissors or rotary cutters, so I don't know how that works with an underlay. I have used small industrial cutting machines with an underlay, so I suspect that it may work fine.
And of course, your scissors need to be sharp and without nicks that could catch on your slippery fabric for the best results.