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Be sure that you wear a respirator mask for fine dust particles when you sand the pieces. You can sand the pieces with a dremel or sandpaper. Start off with a rough grit, and finish with a finer grit. If a stringing hole got filled, you will have to drill it with a dremel. Check the S-hook bars in the ankle and wrist bars to ensure that they haven't been filled in. Purchase these magnets online in a size that matches the holes that you made. Glue the magnets into the holes with super glue. Make sure that the polarities are correct. Do not use regular magnets. They are not strong enough. Buy some small S-hooks from an online bjd shop or a hardware store. Make sure that they are small enough to fit inside the stringing channels, then hook them over the wrists and ankles. Have a large S hook ready for the head. Measure the distance from the left wrist to the center of the chest, double it, then cut a piece of elastic according to that measurement. Next, measure the doll from the top of the neck to the bottom of the torso, double it, and cut another piece of elastic. Tie both pieces of elastic into loops.  The thickness of the elastic depends on the size of the doll and the stringing channels. The bigger your doll is, the thicker the elastic needs to be. Bjds use round (not flat) elastic that is white in color, which you can find in online bjd stores. Thread the shorter piece through the arms and latch it onto the S-hooks. Fold the larger loop in half. Latch it onto the large S-hook, then pull it down through the neck and torso. Pull each half through each leg, then latch it onto the S-hooks too.  Use a pipe cleaner or a bjd stringing tool to string your doll. You can find bjd stringing tools in online bjd stores. If you are confused about stringing your bjd, look up video tutorials online or ask a fellow hobbyist to assist you. You can give the doll a faceup using an online tutorial, or you can commission a fellow hobbyist to do it for you. Purchase clothing, shoes, eyes, and a wig for your doll from a bjd supply. The wig should fit comfortably over your doll's head, but you will need to insert the eyes using poster tack or silicone ear putty.  A faceup refers to the blushing, lip color, lashes, and eyebrows applied to a doll's face. Do not use polymer clay to set the eyes, or you will stain the resin. Be careful what supplies you use for your doll's faceup. Specific brands are recommended for a reason; other brands may melt the resin. If you know how to sew, you can make your doll's clothes yourself. Some people sell bjd patterns, but they may not fit your doll.

Summary:
Sand the doll's seams and do any extra drilling. Add silver neodymium magnets into the doll's head cap. Hook small S-hooks over the bars on the wrist and ankle balls. Cut and tie round, white elastic for the arms and legs. String your doll with the elastic. Give your doll a faceup, eyes, wig, and clothing.