Securing a piece of stock (material you want to cut) to the wasteboard of your CNC can involve a lot of thought and work. The simplest way that I have found to do so, is to sandwich 2 pieces of painters tape with superglue in the middle.

I've read a lot of peoples woes with double sided carpet tape, which is relatively expensive and can be a real pain to remove from the wasteboard.The method I've outlined here, however, is less expensive and is surprisingly strong. Additionally, the stock can be removed when you pull up on a bottom corner of it or when a wedge is forced between it and your wasteboard. Using this method, I've had success cutting wood, acrylic and even aluminum.I should mention that I used fairly shallow cutting depths with aluminum (0.1-0.25mm). Also, I'm only using endmill bits made from carbide (I haven't tried anything else).

Things you'll need:
CNC router

piece of stock (or 2 if you'd like to make a secondary wasteboard to protect the one attached to your machine)

some painters tape (or masking tape)

superglue

Things I'd recommend:

pencil

rubber bands or zip-ties

medium grit sandpaper

Universal Gcode Sender (free software to control your CNC)

Fusion360 (free for hobbyists, students and educators)

Step 1: Make a Squared Guide

If you care about making sure your stock is lined up squarely with your machine, I recommend doing this.

photo1: fix a pencil to your CNC router. You can use a rubber band or zipties for this. For a rubber band, put your pencil through the band, then loop the band around the router/spindle mount and put your pencil through it a second time.

Photo2: load up 'Universal Gcode Sender' and connect your CNC. go to the manual control tab.

Photo3: lower your CNC (pencil) until it touches the wasteboad with the Z- command, making sure not to overshoot it too far. By clicking on the X and Y axis buttons you'll now be able to draw lines on your wasteboard.

Photo4: move your CNC so that it presses up against the Z mount and the router mount (otherwise the pencil might bounce around a bit when drawing the lines).

Step 2: Securing Your Stock to the Wasteboard

Now we'll secure our stock to the CNCs wasteboard.

Pictures 1-3: Add tape to bottom of stock - you can improve hold by roughing up tape with a bit of sandpaper

pictures4-6: Add tape to wasteboard, lining it up with the pencil marks you made in the last step - again you can improve hold by roughing up tape with a bit of sandpaper

pictures7-8: Add glue to tape on wasteboard - zigzag or 'S' patterns are better

picture 9: Line up tape on stock to tape on wasteboard. apply even pressure down on stock. If you didn't do a perfect job lining it up, you can use the router to cut it square if you like.

I'd recommend, if your stock is at all bowed or warped, to do a facing job with your CNC. I won't go into too much detail here, but I do this in Fusion360, and follow this workflow: create a rectangle with the same dimensions as your stock and then go into the CAM section and set up a 2D facing job. If you would like to have a more detailed explanation on how to do this, leave a comment and I'll either make a new Instructable or find a link for a good tutorial.

If you don't care about getting you wasteboard cutup a bit, you can start cutting your stock now. If you'd prefer to use this stock as a secondary wasetboard, and don't mind losing a bit of Z-height, you can repeat the process again with another piece of stock on top of your first. We'll do that in the next step...

Step 3: Repeat the Process With Another Piece of Stock

picture 1: Add tape to bottom of stock - rough up tape a little with sandpaper

picture2: Flip stock over so you can see where your tape needs to go to have it line-up. Then add tape to your secondary wasteboard and rough up the tape with sandpaper

picture 3: Add glue to tape on wasteboard (curvy 'S' or zigzag patterns usually work best)

picture4: Line-up and then apply even pressure over stock.

Now your ready to start cutting!

Going further - One thing I'd like to try, is to cut a grid of holes in a piece of stock (wood) and add threaded inserts (or a thick piece of acrylic with tapped holes) . This way I'd be able to use clamps with the secondary wasteboard.