For this project you'll need a piece of melamine and a couple 2x4s. I'll also assume that you have a router with some kind of base, it can be either a plunge base or a fixed base.
Watch the sled in action:
Step 1: Cut a Slot Into the Melamine
The length of the melamine is determined by the width of the wood you want to flatten. In my case I used a 1200mm strip of melamine in anticipation of any wide slabs in the future.
Find the centre of the melamine, lengthways, and draw a line down the centre. This will be the reference point from now on.
Come in from the ends around 100mm-150mm. This will give you a space to attach stops later. Now drill a through the melamine on each end. This how should be bigger than the router bit that you plan on using. In my case, I was using a 19mm router bit so the hole was 25mm wide.
To cut the majority of the hole I used a circular saw, "dropping" it into the cut. Start the cut away from the drilled holes then come back and finish the off with either a jigsaw or a hand saw.
Step 2: Attach the 2x4s
Use two 2x4s on the top side of the sled. These do 2 things; they stop the router from running into the sides of the slot in the melamine, and brace the melamine, kind of like a spine.
With that said make sure the 2x4s are as straight as possible. If they are curved that will transfer into the melamine.
To attach the 2x4s use screws.
Step 3: Set Up the Rails
Place 2 rails on a flat surface. The height of the rails need to be slightly more than the height of the slab you're flattening, but not so high that the router bit can't reach the wood. In my case I could use more 2x4s.
Join the rails on either end with some scrap wood. This will stop them moving apart as you work.
Now chuck your router into the slot and get milling! This is done by moving the router from left to right taking off around 5mm of material. Take a pass, then move the sled forward up the slab. Take another pass, then repeat. This might take a while but be patient. On my 1 meter long slab it took about 15 minutes per pass.