Done this for this season with Mavic 719 rims and Schwalbe CX Pro tyres.
Kept original rim tape on then built up the rim with electrical tape. You need to get a good seal on the side of the rim. Once you have done that then build up the centre of the rim with tape until it has a good seal too (this helps to air up the tyre).
Now cut a small hole where the valve will go.
I then used a valve out of an old inner tube. It is a good idea to also tape over the valve too as it can be a difficult area to seal.
Once you have done this add your sealant of choice (used Joe no flats but any one would do). If you have your rim built up enough you should be able to inflate tyre with a track pump. If you can't add more tape. Spin the wheel with distribute the sealant around the tyre.
Pump the tyres up to about 40psi and go for a short ride. Leave the wheels overnight to see how they have sealed. My front held air no problem for the start. It took about three rides to get the rear to hold air (as in it would drop about 20psi in an hour long ride but not drop below that).
I think this is due to the sealant taking time to find the smaller leaks. I have had the same with some MTB tyres.
I have been running 40psi up front and 45psi on the rear. About right for my weight on the CX Pros which are quite narrow for a CX tyre. I can take them down to about 30 and 35 but I get enough grip with the higher pressures so don't feel the need.