Well, the only way to make a lager is to, well, make a lager. Fermented with lager yeast at lager fermenting temperatures, and then lagering for about 6 weeks at near freezing temperatures is the classic way to make a lager.
You can make a decent clean tasting lager-like beer, though. I'd use a very "clean" well attenuating ale yeast like Nottingham, and ferment it at 59-60 degrees (NOT ambient temperature- but the temperature of the beer as it ferments. The room might have to be 56-57 degrees). If that's not possible, you could try to do the best you can to keep it under 65 degrees but realize that you may have some fruity esters or other ale yeast characteristics as a result.
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