Done it many times. Simple, but not the most enjoyable job. Very often only the exhaust guides need replacement. First, measure the distance of the old ones from the top of the block down to the guide, intakes and exhausts may be different.
If the camshaft and lifters have been removed, make or buy a suitable drift, get a 4-lb (or so) sledge hammer and drive them down thru the lifter bores and out the bottom. If the cam and lifters are still in place, drive them down about half way until they are just above the lifter, and break the bottom portion off with a chisel (the break off very easily), then drive the remainder out the same way. No. 8 exhaust can be especially challenging because the curvature of the cowl can limit the swing of the hammer. If you can't remove it, an option is to ream it oversize to match an oversize stemmed (special order) valve, and regrind the valve seat after the guide has been reamed. In fact, for every guide you installed you should use a power seat refinisher to make sure the seat is concentric with the new guide bore.
To install, first clean the bores with something like a brass shotgun barrel cleaning brush or equivalent, and drive them in. You'll want a good, firm-fitting drift (K-D #815 if you can find or borrow one) that fits into the guide bore with just slight clearance and has a good shoulder to contact the top face of the guide. Drive to the proper depth. You may have to ream for final clearance to the stem (different clearance for inlet and exhaust), though with the last guides I bought from Egge that wasn't necessary. Freezing them first or packing in dry ice will make installation much easier.
I'd also suggest you read the instructions in the shop manual. This may also be helpful:
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