For Pentaxuser, above.

Most enlarger baseboards are white or grey or woodgrain. If you place the paper on that surface and expose it then you will probably have a fair bit of light spillage, even with masking in the neg-stage - the usual answer would be to put a piece of black card on the baseboard to kill the diffuse reflections. If you have a piece of black card on the baseboard then you cannot easily make alignment marks which will be visible under safelight, which in turn implies that you swing a red filter in to the light path so that you can see what will fall on your paper whilst it is in position and the enlarger is on. Great, except that filters fade or may not be present on a colour enlarger.

Using a dark-red card on the baseboard means that you can use the back of an old print (same size as the paper you are using of course) to decide your paper position, then make soft-pencil marks to show where you want your paper to go for the exposure. Alternatively, use a larger piece of dark-red card with the paper-size drawn on it directly, and move the whole thing around for alignment. Because the card is dark-red it appears light in colour under the safelight and the alignment marks show up clearly, plus any white-light from the exposure bouncing off the dark-red card will have much less affect on the exposure you have made.

That took longer to write than to use the method for an exposure. If you have curly paper then you might be stuck, though you can tape the corners down, use magnets on the base board of a dismantled old easel etc. etc. though this will need some trimming. If the paper is not flat it's probably more practical to use an easel and trim it down afterwards - but if you want to make large prints only infrequently then using the paper without a large and expensive easel can be an economic and practical choice, even if you have to trim a few millimeters to lose the corner marks from tape etc.