How should I design a heat sink for an SMD power mosfet?

Depends on your power levels. From the datasheet you'll get a temperature rise of 62C/W on a minimal copper area, but this goes down to 40C/W with a larger cooling area. A multilayer plane (if you can fit it) with plenty of vias for extra area and copper makes 6cm^2 not all that much. If you look through datasheets for other TOLL-88 packages (or similar, Infineon has weird packages), you'll usually find recommended PCB layouts. At 0.75mOhm rdson, 25C ambient, and Mercury in retrograde you could put 56A through that part before hitting 120C on your junction. The datasheet says you can go as high as 175, but I never take power semis even close to that. (With the minimal copper area you're looking at ~45A to hit 120C).

If this still isn't enough for you, then start looking into heatsinks. The whole point of these packages is to save space, and heatsinks do not do that...

Of course, my analysis only works if you are not switching frequently as it does not take turn-on and turn-off losses into account. It assumes your max ambient is 25C, etc, etc. I'll leave calculating your actual power consumption to you.

You can buy heatsinks that can solder to the PCB copper but, for this device, if you look at the thermal characteristics in the data sheet on page 4, you will see that directly attaching a heatsink to the case is probably you best bet. The typical value of thermal resistance from junction to case is 0.2 degC per watt. Something like this ought to be an option: -

Available from Farnell. There are plenty of options to consider and plenty of offerings from the usual heatsink suppliers.

Cooling from the bottom:

I know from some "Gansystems" applications, that they attach heatsinks on the other side of the PCB. This will require multiple vias in the component footprint to get the heat down to the other side of the PCB. But this strategy is viable, and maybe you have a lot of space on the back.

Cooling from the top:

If you desire 1 heatsink per 1 component then a gluetype heatsink will do. If you require 1 heatsink for multiple transistors the transistors might not be mounted exactly equal in height or planarity. Then go for a heatsink with screw/bolt mount into the PCB with a heatpad to get proper thermal connection.