First off you need two special tools (driver and reamer)that will cost more than a new head and for a pukka job a seat cutting set, that will cost more than an engine, so that might be a barrier.
But if you have or can borrow, support the head on good solid lump of wood, I use an offcut from an old railway sleeper and simply drive out the old guide. Carefully line up and tap in the new guide, the first one I did, I screwed up and got it slightly twisted. Once into the correct depth, use the reamer to open up the guide to the valve stem size. Now either lap in a new valve or first recut the seat to give a nice narrow contact area.
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