Begin by laying out a fondant mat(s), some baking/parchment paper, or dust your counter/worktop with icing/powdered sugar. Then soften the marzipan/almond paste in your hands. Knead it into a rough round shape, & flatten into a disc on your prepared area. Use your hands to round the edges as best as you can, as this will make rolling a circle much easier. Also 500g packet of marzipan is just enough for a thin layer to cover this 8" by 3½" deep cake. So we want to roll a good round shape.
I am using a 2-piece clear silicone rolling mat by Iced Gems. This is the large size & has markings, lines & circles on the top mat, from 6"/15cm to 19"/48cm. Two mats are great as you can turn things over easier. No need for any icing/powdered sugar either. So if not using something similar, you might need the sugar to stop the marzipan sticking to the mat/counter or rolling pin. For marzipan & fondant, if you use a fondant rolling pin, it shouldn't stick either. Alternatively use 2 pieces of paper & a regular rolling pin. You can even mark a circle on your paper and turn over before rolling out, & use as a guide. You want to roll to at least 15" (38cm) wide, to account for also the height of the cake on both sides. Once rolled out, the marzipan will be thin (but not too thin. You can check it against the cake. With a double mat (or 2 sheets of paper), you can lift the marzipan & check it against the cake, making sure it will cover to the bottom of the cake. If not, you can roll out more, if perhaps the marzipan is thicker in the centre. Or, if some ends are too long, you can remove some marzipan & patch it together. Since this will be covered with Royal icing, you can do this & smooth it down.
Next step can be done a few ways, but basically you want to add jam/jelly/preserve to the top & sides of the cake. Traditionally thinned apricot jam is used, but you can use any you have, or a complementary flavour. I used new Aldi Lime & Raspberry Curd (so wasn't even a jam). It was nice & smooth (as well as tasty with a citrus kick), so easy to apply. I spread the curd on top of the cake, then I carefully, using the small cake board, turned the cake over & onto the centre of the marzipan. Alternatively, you can spread the jam on the marzipan centre area, (about 8½-9" or 21½-23cm wide). Make sure you have added the jam to the flat edge of the cake, so your cake 'bottom' will not be as flat an end.
Next step is to then start adding the jam/curd to the sides, & lifting the marzipan up, smoothing out by hand, & letting any excess come over the top, which is really the bottom of the cake. Do a little section (about ¾"/2cm wide, & kind of fan it in towards the top edge of the cake. Do this slowly, & push the marzipan up, & over the folds, using your fingers to smooth the marzipan as best as possible. Using a cupped hand, press the sides to smooth down the folds. You can also tear off any excess marzipan to make the smoothing & joining process easier. Add a little jam at first, lift up a section, smooth & repeat. Once finished, let the excess coming over the top (bottom of the cake), & thin it with your fingertip, making it smooth into & attach onto the cake easier. Any excess jam, wipe away with a clean paper towel as you go.
Next use a cake smoother, the hard type with a handle, or the softer pliable thin ones, & use to smooth the sides of the marzipan covered cake. For marzipan I found the small ones easier. I like to use two (they come in pairs & are inexpensive. Even places like Home Bargains have them), & I basically smooth the sides & the top, with the 2 cards perpendicular to each other. (forming a 90° angle). One on top & one on sides, meeting at the edge of the cake. Then perform a smoothing action. This gives a nice sharper cleaner edge, as well as using a second one ensures you don't mess the marzipan with your finger if holding by hand. And as you will cover the marzipan too, no need to be super precise.
Once happy, turn the cake carefully over, right way up, & onto some parchment/baking paper, on top of a cake board or plate (something to help with turning back over). Then proceed to smooth again, making sure that the flat top edge stays flat. Sides are not as important as covering, but you want the top relatively straight & level. Leave the cake out at room temperature, or even cooler (it is probably winter after all), unwrapped & not in the fridge. We don't want condensation but for the marzipan to dry a little. You can place some baking paper & a clean tea-towel on top if you.
Now you can leave 2 or 3 hours before adding the Royal icing or fondant layer. For British Christmas cakes that are started in October & feed, it is often advised to wait a few days for the marzipan to dry before adding Royal icing, as sometimes it can give the icing a grey tinge to it. However, this quicker cake, was absolutely fine with adding the icing same day (marzipan added in the morning & Royal icing at late afternoon or evening). But to suit your schedule, you can still leave to dry till following day if you need to. Just try & cover the cake with cling film/plastic wrap if leaving for longer than 24 hours.