Making a cross cut sled is really easy and very simple to do. But you do need a little bit of patience and taking your time to align the board to the blade.
The first thing you need to do is cut your rails or runners to fit to your mitre slots. Now every table saw will be different though some of the more expensive saws have a standard. Mine were 19mm slots and 10mm deep. So I cut two strips at 19mm and 8mm. I cut them 8mm so the won't be touching the bottom of the slot once finished, this means it is one less area you have to worry about of friction. Give them a light sand and make sure they slide nicely and freely in the mitre slots with out to much friction or to much sideways movement.
Step 1: Cutting the Base
Once you have your runners made, you need to cut your base of the sled, depending on the size sled you wish to make cut the base to that size. Both larger and smaller have advantages and disadvantages.
Step 2: Find the Centre Line of Base
Find the centre line of the base and set your fence to align with this as if you were to cut down this line. Using washers, or quarters or 10 cent piece or what ever you have on hand place them in the mitre slots so when you place the runners in the slots they are just proud of the table top. Now put glue on the runners and place you base on them using you present fence to find centre. Add some weight and let dry.
Go make a coffee, have a bourbon, make a sandwhich... :-)
Step 3: Tidy the Runners
Cut any extra length of the runners off with flush cut saw. Pre-drill and countersink your holes and screw the runners. Ensure there is nothing that could interfere with the sliding of the sled.
Step 4: Support Boards
Now cut two boards at the length of the sled, making sure they are taller than the height of the table saw blade at full height. This is because these are what hold the sled together.
Put your back board on first, this does not have to be 100% 90 degrees to the blade but if you get it close it will make the sled look better. Glue and clamp getting the even as possible. The screw one end in, allowing you align the other end as even as you wish to go. Make sure you do not put any screws in the path of the blade.
You are now ready to make the first cut, and cut through the back board and stop the saw 3/4 of the way through the base.
With the other board you are ready to do the front board. This is the one you have to get as accurate as possible, being 90 degrees to the blade.
Repeat the process you did with the back board, use a square to align the board to the blade and then reclamp to add final screws. Becareful when clamping asit can move the board, it can slide on the glue. Once you have the it screwed in recheck to make sure you still have it aligned to 90 degrees to the blade.
Step 5: Safety Block
Add a safety block behind the back board to act as a blade guard when cut through the back board.