Loads of good pointers so far. Here is my addition.
You can get old tyres to work as tubeless, even ghetto tubeless. Check them for cuts and tears carefully.
Inflate with a tube installed to seat the bead on the rim. Break one bead to get the tube out and to re insert the removable valve stem. brush plenty of soapy water over the bead and get some all over the spokes as you may be leaking here and round the valve stem. Take the core out of the valve stem, it is nigh on impossible to get enough air in quick enough with the core fitted IME. Pump up to reseat the bead. finger over the valve stem is fine for initial checking of leaks/bubbles. Check where the bubbles are coming from, anything untoward? When you are happy that it is generally air tight you can get some Stans etc in through the valve stem. The clean nozzle from a caulking tube works ok as a funnel. Re insert the valve core and re inflate.
Other tips are to put a tube round the outside of the tyre like a big rubber band to help with initial inflation to seat the bead.