Fit your tyres dry onto the rims and leave inflated for a few days. Take them off and refit a couple of times to get them to stretch a bit. Scrape/dig out as much old dried up and powdery cement from the rim as you can, especially the big lumps etc. and then wire brush to leave a basically flat, clean(ish) and dry surface. Apply a coat of your favourite rim cement (if you can still get it, mine is/was Dunlop!) to rim and let it dry (overnight is good). Apply another thin coat to rim and tyre base tape and let go tacky and then fit tyre. Inflate a bit to get shape and centre the tread, roll wheel on floor with a bit of pressure to try and get contact all round and then inflate. Clean any excess cement from the side of the rim and tyre with a bit of solvent. Mind you, if you've stretched the tyre properly first and been careful this shouldn't be necessary. In theory.
Or use tub tape and follow instructions - but you still need to clean etc. the rim first.