It really depends on what clown and what anemone and whether the type of anemone is the clowns natural host, and also whether the clown is tank raised - a wild caught clown is more likely to host, tank raised I have heard hardly ever do.
I have a pair of Ocellaris clowns, wild caught, and I have a Bubble Tip Anemone in my tank, the BTA is not the Ocellaris clowns natural host, although they do tend to host them over time.
I put my clowns in the tank and after a week of them not hosting, I decided to get a net and kind of corner them towards the anemone, and made them stay near it for 5mins, and then just sat the net in the tank and they stayed put, i kept up with this until the clowns would stay where they were, hosting the anemone, this took 4 hours of being persistent and when ever they left the space around the anemone, i got the net and moved them towards the anemone (after a while as soon as they saw me come to the tank and they were away from the anemone, they swam back to it

).
Other ways are to put pictures up of similar clowns hosting the same anemone on the side of the glass, I used this trick on top of the above. once they saw the pictures they seemed to get the hint a little quicker.
There are a range of other methods, but i found this was less stressful to both clowns and anemones as the anemone did not need to be disturbed without the clown naturally trying to host, and that the clowns were not forced to actually touch the anemone.